Showing papers in "Wear in 1994"

Comprehensive study of parameters for characterising three- dimensional surface topography: III: Parameters for characterising amplitude and some functional properties

Abstract: It is recognised that profiling techniques have been widely used in industry and academic research for manufacturing control and functional control of surface roughness, in some cases, however, the profiling techniques and two-dimensional (2D) parameters defined in standards are inadequate and/or unsuitable for characterising surfaces. In recent years, there has been an increasing need for the characterization of surface topography in three dimensions (3D). Therefore a clearly defined, effective and widely accepted 3D parameter set is urgently required. Based on an understanding of intrinsic properties of surface topographic features and parameters, a primary 3D parameter set is proposed in the last two parts of this series of reports. Some of the parameters included in the primary parameter set are extended from their 2D counterparts; others are specifically defined for characterising 3D surface topography. Evidence is given to support the reasons for inclusion of each parameter in the set. In this paper, the definitions and algorithms of amplitude and some functional parameters are presented. The feasibility in terms of sampling conditions, and the application scope of the parameters is discussed. Not only is the mathematical/statistical background of the parameters presented but also experimental results obtained by testing a wide range of engineered surfaces are provided to justify the proposal. The lessons (parameter rash, correlation of the parameters) obtained in defining 2D parameters are taken into consideration in defining 3D parameters.

Comprehensive study of parameters for characterising three-dimensional surface topography: IV: Parameters for characterising spatial and hybrid properties

Abstract: This is the last part in a series of reports describing a comprehensive study of parameters to characterise three-dimensional surface topography. In conjunction with other parts of the report, this paper deals with parameters used to characterise spatial and hybrid properties of surface topography in three dimensions. Some statistical and signal processing techniques, e.g. two-dimensional autocorrelation function and spectral analysis, which are used to extract spatial and hybrid parameters, are introduced in the paper. Some spatial and hybrid parameters are recommended to be included in a primary set to serve for a comprehensive characterization of surface topography in three dimensions. The definitions and algorithms of the parameters are given in the paper. The feasibility and significance of the parameters in characterising surface topography in three dimensions are addressed by conducting experiments on a wide range of engineered surfaces. All discussions in the paper are based on theoretical and experimental justification.

Control of friction

Abstract: In kinematically constrained sliding and/or rotating applications, the frictional force can reach very high values if surfaces cannot be lubricated with either liquid or solid lubricants. This situation arises in many bearings or machines where lubricant cannot be applied because of the unacceptability of airborne contaminants or because of the high operating temperature of machine components. When the lubricant is absent the frictional force increase is primarily due to the plowing of the interface by wear particles which eventually leads to seizure. Frictional force in bearings and sliding surfaces, even in the absence of lubricants, can be maintained at low values when the plowing by wear particles is eliminated by making the surface undulated to trap wear particles. Frictional force cannot be eliminated completely because of the presence of microscopic plowing that occurs even with the undulated surface. The friction coefficients obtained with the undulated surfaces without lubricants were as low as those found in boundary lubrication of flat surfaces. This indicates that the primary role of liquid lubricants is to reduce plowing through the elimination of wear particles from the interface and also through the minimization of particle agglomeration.

Wear and seizure of binary Al---Si alloys

Abstract: The wear and seizure behaviour of binary Al-Si alloys containing up to 23% Si has been investigated in sliding against a hard steel counterface by continuous loading experiments carried out in a pin-on-disc machine. Addition of silicon to pure aluminium improves wear and seizure resistance. The wear characteristics of alloys containing 7-17% Si, when considered as a function of load, exhibit distinct mild wear, severe wear and seizure regimes. Mild wear is characterised by the in situ formation of a protective iron-rich compacted layer. The severe wear is initiated when the protective layer is removed as a result of subsurface flow. On further increment of load this leads to seizure. Based on morphological observations in SEM, a qualitative model has been proposed for the wear behaviour of this group of alloys. An alloy containing 23% Si exhibits only mild and severe wear regimes and does not show any seizure within the present experimental limits. The alloy has low thermal conductivity and is prone to plastic flow instabilities as observed in compression tests. Because of these factors, the critical contact temperature required for seizure is not attained under the test conditions.

Film thickness in elastohydrodynamically lubricated elliptic contacts

Abstract: A multilevel solver for the circular contact was extended to elliptical contact problems. After verification of its predictions by comparison with results presented in literature, it was used to study the variations of film thickness with varying operating conditions and aspect ratio of the contact ellipse. Detailed computational results are presented and observed tendencies are traced back to the modelling equations. Subsequently it is demonstrated how and when, for contacts with the entrainment directed perpendicular to the major principal axis of the contact ellipse, the pressure and film thickness on the centre-line of the contact can be predicted accurately from an equivalent line contact analysis. Finally, survey graphs of the minimum and the central film thickness are presented and a formula is given that predicts the central film thickness as a function of load and lubricant parameters, and the ratio of reduced radii of curvature of the surfaces. This formula incorporates asymptotic behaviour and as a result it can be applied for all conditions. In particular, its accuracy for contacts with the major principal axis of the contact ellipse perpendicular to the entraining direction is demonstrated in this paper.

Wear characteristics of Al-Si alloys

Abstract: The wear characteristics of Al-Si alloys containing 2–20 wt.% have been studied using a pin-on-disc type wear testing machine at room temperature. The effects of alloy composition, sliding distance, sliding speed and load on wear rate of Al-Si alloys have been investigated. It has been found that the wear rate is strongly dependent on alloy composition, applied load and sliding speed. The wear rate decreases and the load-bearing capacity of the alloy increases with increasing silicon content. The nature of the wear process changes with alloy composition and experimental conditions.

The solid particle erosion of polymer matrix composites

Abstract: The solid particle erosion behaviour of four different types of polymer matrix composites reinforced with glass fibres have been characterized. The erosion rates of these composites have been evaluated at two impact angles (90° and 30°) and two impact velocities (38 and 45 m s −1 ). The erosion response, erosion efficiency and the erosion micromechanisms of these composites are presented and discussed in detail and also compared with the available data in the literature on similar materials.

A model for the abrasive wear resistance of multiphase materials

Abstract: A theoretical model for estimating the abrasive wear resistance of multiphase materials and composites from the wear resistance of the constituents is proposed. Starting from Archard's equation, two rules for the dependence of the amount of reinforcing phase, corresponding to two wear modes, have been derived. These rules of mixture, constitute estimations of the upper and lower limits for the wear resistance of a composite. The theory also results in two corresponding formulations of the wear equation generalised to multiphase materials. The applicability and limitations of the theory are discussed. The validity of the theory is supported by results from two test series. Three-body abrasive tests with silicon carbide abrasives were performed on simple specially fabricated model composites and silicon carbide particle-reinforced aluminium alloy composites were tested in two-body abrasion against silicon carbide and flint abrasives. The model serves to clarify the roles of the matrix and the reinforcing phase of a composite in resisting wear. The model can be instrumental in estimating upper and lower limits of wear and in indicating how to redesign a composite or modify a tribosystem (changing load, filtering abrasive particles, etc.) to transform the wear of a multiphase material towards the least severe mode.

Correlating tool wear, tool life, surface roughness and tool vibration in finish turning with coated carbide tools

Abstract: Experiments have been carried out in an attempt to monitor the change of workpiece surface roughness caused by the increase of tool wear, through the variation of the vibration in finish turning, under different cutting conditions. The vibration was measured by two accelerometers attached to the tool and the parameter used to make the correlation with surface roughness was the r.m.s. of the signal. The tool of one experiment was photographed at different stages of the cut in order to explain the wear formation and the behaviour of surface roughness as the cutting time elapsed. The material machined was AISI 4340 steel and the tool was coated carbide inserts. The results show that vibration of the tool can be a good way to monitor on-line the growth of surface roughness in finish turning and, therefore, it can be useful for establishing the end of tool life in these operations. Another conclusion is that, when coated tools are used, the behaviour of surface roughness as cutting time elapses is very different from that when uncoated tools are used.

An investigation into the mechanisms of closed three-body abrasive wear

Abstract: Contacting components frequently fail by abrasion caused by solid contaminants in the lubricant. This process can be classified as a closed three-body abrasive wear process. The mechanisms by which trapped particles cause material removal are not fully understood. This paper describes tests using model elastohydrodynamic contacts to study these mechanisms. An optical elastohydrodynamic lubrication rig has been used to study the deformation and fracture of ductile and brittle lubricant-borne debris. A ball-on-disk machine was used to study the behaviour of the particles in partially sliding contacts. Small diamond particles were used as abrasives since these were thought not to break down in the contact; wear could then be directly related to particles of a known size. The particles were found to embed in the softer surface and to scratch the harder. The mass of material worn from the ball surface was approximately proportional to the particle sliding distance and abrasive concentration. Small particles tumbled through the contact, whilst larger particles ploughed. Mass loss was found to increase with abrasive particle size. Individual abrasion scratches have been measured and related to the abrading particle. A simple model of the abrasive process has been developed and compared with experimental data. The discrepancies are thought to be the result of the uncertainty about the entrainment of particles into the contact.

Finite-elements model for the contact of rough surfaces

Abstract: In this paper the elastoplastic asperity-based model for the contact of rough surfaces is presented. The model adopts most of the basic asperity-based model's assumptions, introducing, however, a more realistic elastoplastic deformation law for the analysis of individual asperity deformation. The elastoplastic deformation of an individual asperity is numerically investigated by means of the Finite Elements Method, and evaluated relations are combined with statistical description of the surface. The results were contrasted and discussed together with results obtained from other models, as well as with experimental results.

Microstructural effects on the sliding wear resistance of a cobalt-based alloy

Abstract: The influence of the microstructure on the dry sliding wear resistance of a hypo-eutectic Stellite 6 alloy was investigated under conditions leading to severe metallic wear of the hardfacing alloy. Conventional chill casting as well as laser surface cladding were used to produce a wide range of solidification microstructures. The hardness of the alloy was strongly dependent on the microstructure and in particular on the size of the dendrites. Under the sliding conditions investigated, severe delamination wear of the Stellite occurred. High coefficients of friction were measured and the structure in the subsurface was completely destroyed by the resulting stress cycles. During the stationary wear regime, no dependence of the wear rate on the as-solidified microstructure could therefore be determined. However, a strong influence on the wear resistance of alloying elements which affect the matrix properties was observed. Suggestions are made for the improvement of the wear resistance of such alloys under similar sliding conditions.

The effect of sliding velocity on the friction and wear of UHMWPE for use in total artificial joints

Abstract: The effect of two different sliding velocities on the friction and wear of ultrahigh molecular weight polyethylene (UHMWPE) has been investigated in a tri-pin-on-disc apparatus using bovine serum as the lubricant. UHMWPE pins were slid at velocities of 35 and 240 mm s − 1 on stainless steel counterfaces with the surface roughness R a being varied in the range 0.014–0.078 μm. The coefficient of friction was found to be in the range 0.07–0.2 and was not dependent on the sliding velocity. The highest friction values were found with a counterface roughness R a of 0.042 μm. Sliding velocity had little effect on the wear factor. For smooth counterfaces with a surface roughness of less than 0.05 μm, all the wear factors were less than 1.3 × 10 − 8 mm 3 Nm and there was no significant difference in the wear factors produced for the two different sliding velocities. An increase in the counterface roughness to between 0.07 to 0.08 μm increased the wear factor by over twenty times to a value greater than 3 × 10 − 7 mm 3 N − 1 m − 1 . For these rougher counterfaces a statistically significantly higher wear factor was found for the lower sliding velocity. The results show clearly that variation in the sliding velocity has only a small effect on the wear of UHMWPE compared with the changes found when the counterface topography was altered. The results indicate that it is reasonable to accelerate tri-pin-on-disc wear tests by increasing the sliding velocity within the range specified, but it is essential to control the topography of the counterface.

Fretting in orthopaedic implants: A review

Abstract: Metallic implants in the human body are widely used in surgery, and their corrosion and degradation processes are being extensively investigated around the globe. As with other structural components, possible mechanisms of failure in orthopaedic implants are: mechanical fracture, wear and corrosion, or, more importantly, combinations of these factors. Many implants are exposed to high loads and intense wear, which, as a consequence of the patient's movements, are repeated an immense number of times. The consequent degradative effect on the metals is greatly increased by the fact that the surrounding body-fluid environment is corrosive. Considering the high human cost of revisional surgery and the fact that even “small” amounts of corrosion products released into the body tissue are unacceptable, it is clear that the nature and distribution of corrosion products released into the body from orthopaedic implants remains an important issue. Modular implants are being developed, and fretting corrosion/wear are potential degradation problems. They play a deleterious role in the degradation process of the articulating implants by producing fretting debris, which can be detrimental to body tissues and will significantly reduce the performance of the implants. Therefore, this paper reviews the mechanisms of fretting and fretting fatigue that accelerate the fatigue failure of orthopaedic implants, and also presents a state-of-the-art review of the investigations carried out by many workers on this subject.

A fundamental tribochemical study of the third body layer formed during automotive friction braking

Abstract: The chemical nature of the transfer film (TF) or third body layer (TBL) formed at the friction interface during automotive friction braking has been studied in detail using energy dispersive X-ray (EDX) and X-ray photoelectron spectroscopic (XPS) analytical techniques. Optical and scanning electron optical methods have been used to examine the friction surfaces of the two elements of the friction couple, grey cast iron and asbestos free friction materials. Evidence is presented which relates the friction characteristics of the couple to transfer film chemistry and friction material composition. The role of lubricants and abrasive additions to the friction material and their effectiveness at a range of contact pressures/temperatures, is examined in detail. Information obtained from imaging X-ray photo-electron spectroscopy studies will be used to show that transfer film chemistry is dependent upon asperity temperature/contact pressure and the composition of the friction couple components. These data are used to explain changes in friction coefficient which may occur during automotive braking. The influence of transfer film chemistry upon wear rate of the friction couple components is also discussed. Friction performance was found to be independent of transfer film thickness or topography but sensitive to transfer film composition. The chemistry of the transferred layer was influenced by the morphology and mechanical properties of the friction material abrasive. Friction material wear rates and friction coefficient were not interrelated.

Mechanism of rubber abrasion. Part I: Abrasion pattern formation in natural rubber vulcanizate

Abstract: The driving force to generate the periodic surface patterns, and thus rubber abrasion consists of two kinds of periodic motions, stick-slip oscillation and the microvibration generated during frictional slidings of rubber. The stick-slip oscillation is the driving force to propagate cracks, then abrasion patterns and the microvibration with the natural frequency of the rubber induced in the slip phase of the stick-slip oscillation is another driving force for the initiation of the cracks. Although initial cracks originate in the slip region of the rubber surface, the propagation of the cracks is strongly excited in the stick region. Accordingly, the initial size of the abrasion pattern, pattern spacing, equals the distance determined by the natural period of the rubber and the mean sliding velocity while the constant pattern spacing after the critical number of frictional slidings agrees with the distance given by the period of the stick-slip oscillation and the mean sliding velocity. Consequently, during rubber abrasion, two driving forces produce bimodal size distribution of abraded particles, small particles of the order of ten micrometres by microvibrations and large ones of the order of a few hundred micrometres by the stick-slip motions.

Some frictional features associated with the sliding wear of the nickel-base alloy N80A at temperatures to 250 °C

Abstract: The time-dependent variations of friction coefficient and the contact resistance of a nickel-base high-temperature alloy, N80A, during like-on-like sliding in pure oxygen at temperatures of 20–250 °C were simultaneously recorded and the tribological behaviour correlated with the nature of the sliding contact. A transition to a positive contact resistance always occurred after some time of sliding in the temperature range investigated. However, the time-dependent variations of coefficient of friction showed quite different features at the various temperatures. Corresponding to the times of the transitions in contact resistance, at 20 °C, the friction coefficient increased to a higher value from the initial value while, at 250 °C, it decreased to a lower value; however, at 150 °C, the coefficient of friction remained unchanged after the transition. Scanning electron microscopy observations have shown that the predominant factor for such different tribological responses at the various temperatures is the nature of the contact between the surfaces. At 20 °C, the real contact areas mainly consisted of loosely compacted particles while, at 250 °C, the load-bearing areas were smooth wear-protective oxide layers. At the intermediate temperature, 150 °C, the load-carrying areas comprised both types of contact. A mathematical model has been proposed and is used to relate the frictional behaviour to morphological features of the wear surfaces developed at the various temperatures.

The dynamic analysis of stick-slip motion

Abstract: The most widely accepted cause of stick-slip motion is that the static (μ s ) exceeds the kinetic friction coefficient (μ k ), or that μ k drops rapidly at small speeds. Using a dynamic analysis it is shown that the rate of increase of μ s ( t ) with sticking time is a crucial parameter in addition to the condition of μ s > μ k , and that stick-slip may occur even if μ k increases with speed V . A general equation is derived which describes the stick-slip amplitude (χ s ) in terms of substrate speed V o , spring stiffness K and damping γ, for an arbitrary μ s ( t ) and a linearized μ k ( V ), in contrast to a set of previous equations derived by Brockley et al . for an exponential μ s ( t ) and a linearized μ k ( V ) [1]. Additional equations are developed for the “saturation” speed ( V ss ), below which χ s is independent of V o , and also for a critical substrate speed V c above which the stick-slip amplitude vanishes. At speeds between V ss and V c the stick-slip amplitude generally decreases with increasing V o , K and γ. Depending on the detailed conditions, different sliding modes including smooth sliding, near-harmonic oscillation or stick-slip can result. Equations developed in this paper suggest practical methods of reducing or eliminating stick-slip for a general system.

The roles of hardness in the sliding behavior of materials

Abstract: It is generally recognized that hardness is one of the key factors which influence the sliding behavior of different materials combinations. However, in many discussions the only hardness value considered is that of the softer of the two materials in a tribological pair. This is usually the case when a simple linear wear equation (Holm, Archard, Khruschov) is cited. Observations on many materials combinations demonstrate that the effects of hardness are much more complex. Hardness varies with position and time. It can depend on temperature, sliding speed and the chemical environment. The sign of hardness gradients adjacent to the sliding surface affects sliding behavior. Transfer and subsequent mechanical mixing strongly influence local hardness. Changes in hardness can affect transitions in friction and wear. Relative hardness values can help to explain differences in debris and in smooth and rough sliding. They can also help us to understand geometric effects such as those noted when materials are interchanged in a test system. Examples will be described.

Machining characteristics of titanium with rotary electro-discharge machining

Abstract: Where force suction flushing is not feasible, alternative methods to clear the debris effectively from the gap between the electrodes in electro-discharge machining (EDM) have been attempted, e.g. the application of a magnetic field, special planetary motion of the tool electrode or low frequency vibration of the electrode. The orbital motion of the electrode helps in improving the machining rate. It also improves, to some extent, form and dimensional accuracy in the machining process. This paper presents the experimental investigation of EDM of titanium alloy with a copper-tungsten electrode in respect of metal removal rate, electrode wear, surface roughness by varying currents and electrode rotations. An effort has also been made to compare these results with those of stationary electrodes. Experiments were statistically designed, and a mathematical model of the process has been developed. It is found that rotating the electrode improves the metal removal rate due to improved flushing action and sparking efficiency. However, this results in high surface roughness. The electrode wear rate also increases with increasing speed. However, the wear ratio is not significantly affected.

Friction behaviour of TiN, CrN and (TiAl)N coatings

Abstract: A series of experiments have been carried out, using a ball-on-disc testing machine, to investigate the sliding friction behaviour of physically vapour-deposited TiN, CrN and (TiAl)N coatings against steel under both dry and lubricated conditions, employing various applied loads and sliding speeds. All the friction coefficient vs. sliding distance curves were characterised by an initial transient state where the friction coefficient either increased (dry condition) or decreased (lubricated condition) rapidly, followed by a steady state. Experimental results revealed that ploughing actions of the hard asperities of the coating surface and material transfer from the steel counterface to the coating surface played an important role in determining the friction behaviour during the initial transient state and steady state respectively. Among all the coatings tested, the (TiAl)N coating, which possessed the highest hardness and surface roughness, exhibited the highest friction coefficient under both dry and lubricated conditions. Lubrication significantly reduced the friction coefficient of all coatings investigated.

Galling mechanisms in sheet forming operations

Abstract: The paper reviews some important observations concerning the initiation of galling for various combinations of tool and sheet materials. The experimental conditions have been chosen to resemble the contact conditions in sheet metal forming. Typically, a hard and smooth tool surface is repeatedly in contact with a soft and rough sheet surface. The plastic wave is introduced as a general mechanism for subsurface plastic working of the contact asperities on the sheet during friction contact. The same mechanism is used to describe the transfer of sheet material to the tool surface. Generally, different types of tool surface defects act as traps for sheet fragments which stick to the tool surface. Typical defects are grinding scratches and holes of various origin. A possible initiation from transfer aluminium oxide to the tool surface for hot-dip galvanized sheet steels has also been observed. The action of defects as initiation sites for sheet material transfer seems to be virtually independent of the material combination, as well as the presence of lubricants. However, the further build up of the transfer layer and formation of big lumps of sheet material on the tool surface is very sensitive to the material combination and the presence of lubricants. Special initiation mechanisms (e.g. aluminium oxide) can, however, be observed in action together with the defect mechanisms.

Wear of hot rolling mill rolls: an overview

Abstract: Rolling is today one of the most important industrial processes because a greater volume of material is worked by rolling than by any other technique. Roll wear is a multiplex process where mechanical and thermal fatigue combines with impact, abrasion, adhesion and corrosion, which all depend on system interactions rather than material characteristics only. The situation is more complicated in section rolling because of the intricacy of roll geometry. Wear variables and modes are reviewed along with published methods and models used in the study and testing of roll wear. This paper reviews key aspects of roll wear control — roll material properties, roll pass design, and system factors such as temperature, loads and sliding velocity. An overview of roll materials is given including adamites, high Cr materials, high speed tool steels and compound rolls. Non-uniform wear, recognised as the most detrimental phenomenon in section rolling, can be controlled by roll pass design. This can be achieved by computer-aided graphical and statistical analyses of various pass series. Preliminary results obtained from pilot tests conducted using a two-disc hot wear rig and a scratch tester are discussed.

Fade and wear characteristics of a glass-fiber-reinforced phenolic friction material

Abstract: The fade and wear characteristics of a glass-fiber-reinforced friction material were studied using a Chase friction material testing machine. At low counterface temperatures, the friction material showed relatively high friction in the range 0.4–0.5. During fade tests, the coefficient of friction dropped to about 0.18 at 343 °C. Re-conditioning the wear surface at the end of a fade test altered the frictional behavior during a subsequent fade test. The wear tests showed that the specific weight loss per unit load and sliding distance decreases with increasing applied load and speed, but increases with increasing bulk drum temperature. At high temperatures, thermochemical degradation and fiber pull-out appear to contribute to higher specific wear rate. The worn surfaces of the specimens were observed by scanning electron microscopy and analyzed by energy-dispersive X-ray analysis. The results were consistent with low friction coefficients due to film formation on the worn surfaces of glass fiber at high temperatures. This film could be removed at lower temperatures by either sliding (application) or by sanding.

A method for optimizing the particle flux in erosion testing with a gas-blast apparatus

Abstract: The erosive wear rate of a material depends on the flux of particles striking the surface, and in order to measure an erosion rate of a material in which each particle strikes the surface independently of all other particles, as is generally assumed, experiments should be carried out at low particle fluxes. However, the flux used in testing is often chosen arbitrarily with little rational basis for the choice. The mechanisms have been studied by which the erosive conditions depend on particle flux in a gas-blast erosion rig. The occurrence of particle-particle interactions in the region between the nozzle and target, mainly between incoming and rebounding particles, depends strongly on the flux. A novel method has been used to determine the extent of particle-particle interaction, and to determine the flux below which the effects of particle-particle interactions are insignificant. The method facilitates the generation of accurate and meaningful erosion data, within the shortest practicable time.

The erosion-oxidation behavior of HVOF Cr3C2-NiCr cermet coating

Abstract: DenSys DS-200 coating is a proprietary hypersonic velocity oxygen fuel (HVOF) Cr 3 C 2 -NiCr cermet coating used in elevated temperature service environments including fluidized bed boilers, coal-fired boilers and municipal waste incinerators. The elevated temperature erosion-corrosion (oxidation) behavior of this HVOF Cr 3 C 2 -NiCr coating was investigated using a range of test temperatures, impact angles, erodents and particle velocities. The erosion-corrosion behavior of HVOF Cr 3 C 2 -NiCr coatings was compared with those of 1018 steel and other thermal-spray coatings including FeCrSiB (Armacor M), Ni-base, Cr 2 O 3 -6SiO 2 -4Al 2 O 3 (Rokide C), Cr 2 O 3 -12SiO 2 -2Al 2 O 3 -4MgO (Rokide MBC) and WC-NiCrCo (SMI 712). It was found that the erosion-oxidation resistance of HVOF Cr 3 C 2 -NiCr coating was higher than those of 1018 steel and other coatings at impact angles of 30° and 90°. The erosion-oxidation behavior of coatings was well related to their morphology. The high erosion-oxidation resistance of the HVOF Cr 3 C 2 -NiCr coating was attributed to its low porosity, fine grain structure and homogeneous distribution of hard carbides/oxides which form a skeletal network within a ductile and corrosion-resistant metal binder.

An analysis of the normal bouncing of a solid elastic ball on an oily plate

Abstract: An analysis is presented of the impact of a solid, elastic ball on an elastic, semi-infinite solid covered by a thin layer of lubricant. The ball is decelerated by very high hydrodynamic pressures of GPa proportions, which also create substantial elastic deformations of the solids as the film thicknesses fall to micrometre or sub-micrometre proportions. The time of the impact is only slightly longer than that predicted by classical Hertzian contact analysis. The central dimple formed in the equivalent elastic ball near a rigid plane maintains a near constant film thickness as the impact proceeds, although the radius of the conjunction grows and then decays rapidly in this dynamic process. The minimum film thickness occurs in a ring whose rapidly changing radius is closely predicted by Hertzian dry impact analysis. The viscous damping plays a modest role in the case considered, while the ball rebounds to 95.7% of its original drop height. The pressure-time traces in the centre, and indeed at most locations in the impact zone, exhibit a remarkable second pressure peak, or spike, to yield a profile very similar to the familiar pressure-distance trace from steady state, entraining elastohydrodynamic lubrication problems. The findings thus confirm the experimental observations of such pressure spikes reported in 1985 by Safa and Gohar ( Proc. 12th Leeds-Lyon Symp. Tribol ., 1985). The new solution procedure outlined in the paper is expected to be useful in analysing bouncing ball arrangements, used primarily in Sweden, for rheological studies of oils.

Mechanism of rubber abrasion: Part 2. General rule in abrasion pattern formation in rubber-like materials

Abstract: The mechanism of rubber abrasion proposed in the previous paper was reconfirmed not only in unfilled and filled NR but in SBR and silicone rubber. It is an essential and general rule in rubber abrasion that the microvibration induced in the slip phase of stick-slip oscillation generates the initial abrasion patterns and the stick-slip motion propagates them to the final abrasion patterns. The initial pattern spacing agrees with the distance determined by the relation between the natural frequency of the material and the mean sliding velocity, meanwhile the final pattern spacing in the propagation reaches the distance given by the frequency of the stick-slip oscillation and the mean sliding velocity. Reinforcement by carbon blacks makes the frequencies of the both periodic motions in rubber larger, which of course gives the smaller initial and final pattern spacings in rubber abrasion. In addition, the microvibration attenuates more rapidly in more filled rubbers when it spreads over the rubber surface as a surface wave. Both phenomena have a great influence on the smaller abrasive wear of more filled rubbers.

The role of porosity in the dry sliding wear of a sintered ferrous alloy

Abstract: Porosity is the distinguishing feature of sintered materials and has a negative effect on their mechanical properties. The influence of porosity on the wear behaviour of materials depends on the wear conditions and is not clearly identified. In the present investigation, the evolution of the dry sliding wear resistance of a sintered Distaloy AE with increasing total vol.% porosity (in the range 9–22 vol.%) was studied. Under certain conditions and depending on the pore size, porosity was observed to be beneficial for wear resistance by entrapping the wear debris and preventing the formation of large abrasive agglomerates. Pore filling superficially reinforced the material and resulted in diminishing plastic deformation and particle detachment around the pores.

Wear behaviour of plasma-nitrided martensitic stainless steel

Abstract: The wear behaviour of plasma-nitrided martensitic stainless steel, AISI 440C, has been investigated under various contact conditions including rolling, sliding and combined rolling-sliding, at a wide range of applied loads. It was found that plasma nitriding was effective in improving the wear behaviour of the steel and wear of the nitrided steel occurred in a mild mode under all testing conditions. In contrast to the unnitrided steel which underwent a change from mild wear to severe wear at high loads due to surface and subsurface cracking and shearing, the plasma-nitrided steel did not show such a transition over the load ranges tested. Oxidative and delamination wear dominated the wear process of the nitrided martensitic stainless steel. The short-time nitrided layers exhibited better wear resistance than the long-time nitrided layers produced at similar temperatures.