Showing papers in "Surface Engineering in 1998"

Current trends in laser shock processing

Abstract: The physical and mechanical aspects of laser shock processing (LSP) with plasma confinement were investigated with the aim of establishing the state of the art and assessing possible applications. Using a pulsed Nd :glass laser, fundamental processing aspects were studied, such as the precise role played by surface protective coatings and laser impact area on shock loading. For instance, it was shown that Al based coatings could enhance the peak stress values experienced by iron based surfaces by up to 50%. Residual stresses were investigated as the main contributor for improving surface behaviour. Lastly, the beneficial effects of LSP were demonstrated through two applications of the process: improvement offatigue behaviour and pitting corrosion resistance of steels.

IMPROVED Ti1–xAlxN PVD COATINGS FOR DRY HIGH SPEED CUTTING OPERATIONS

Abstract: Titanium aluminium nitride (Ti1–xAlxN) has been shown to enhance the performance of cutting tools in dry, high speed machining of cast iron and die steels when compared with TiN and TiCN coatings. The ultimate benefit of Ti1–xAlxN coatings depends on two main criteria: the microroughness of the coating surface has to be minimised, and the oxidation resistance has to be maximised. The microroughness is governed by the choice of deposition method whereas the oxidation resistance is controlled by the composition of the coating. In particular, the generation of macroparticles during the PVD process has to be suppressed and the Al content of the film metallic constituents has to be kept between 40 and 60 at.-%. To achieve these demands supplementary to excellent adhesion, a PVD deposition process has been developed based on combined steered arc and unbalanced magnetron sputtering techniques. Using Cr in the steered arc sustained metal ion etching process the desired adhesion criteria may be exceeded an...

Effect of process parameters during friction coating on properties of non-dilution coating layers

Abstract: Few surface modification processes appear to coat a 1 mm thick compact layer on a substrate successfully, although several technologies have been proposed and applied. The present paper concerns a fundamental approach to establishing a friction surfacing technique that produces a 1mm thick hard coat. Experiments were carried out to clarify the effect of process parameters on the quality of coatings. Martensitic stainless steel with 1C-17 Cr was the coating material and low carbon structural steel the substrate, using a conventional friction welding machine equipped with a sliding stage. A 20 mm wide and 1 mm thick stable hardened layer was produced using a 20 mm dia. Rod of coating material. The dimensions of the layer depend on the rotational speed of the consumable rod, as lower rotational speeds produce thicker, wider coating layers. Calorimetric measurements revealed that the total heat input rate was not influenced by the frictional speed, however the ratio of thermal distributions on to the ...

Investigation into application of electrodischarge machining as steel surface modification technique

Abstract: The feasibility of surface hardfacing/alloying of tool steels using the EDM process has been studied. By this technique, material transfer from the tool electrode (made of sintered Fe–WC) to the steel substrate took place, leading to the formation of a surface alloyedlayer consisting of antiwear constituents (e.g. undissolved carbides). The parameters of the EDM process have been optimised and the microstructure and surface properties (roughness, microhardness, and wear resistance) of the treated material studied. Finally, the industrial potential of this technique is discussed.

Improving wear and corrosion resistance of thermal sprayed coatings

Abstract: Thermal sprayed coatings are used in various applications often dedicated to corrosion and wear protection.Poresand other coating imperfections reduce corrosion and wear resistance and/or necessitate higher coating thicknesses. To improve the quality of thermal sprayed coatings, various post-treatment processes can be used. Characterisation of the efficiency of sealing was studied by metallographic examination andcomparative investigation of the corrosion and wear behaviour of the thermal sprayed coatings used with and without sealing and also by measuring the insulation resistance of the coating system. The results differ among the sealants. It is possible to remove the sealants from the coatings by mechanical treatment. Corrosion tests of the sealants demonstrate their efficiency in closing the interconnected pores and, consequently, preventing corrosion.

Wear behaviour of diffusion and compound layers in nitrided steels

Abstract: The wear behaviour of two nitrided steels was studied in order to evaluate the tribological properties of the compound layer and of the diffusion layer. The specimens were treated in different conditions, in order to obtain different microstructural and compositional characteristics of the nitrided surfaces. Concerning the diffusion layer, microstructural homogeneity tends to prevail over the microhardness profile in affecting wear resistance, since grain boundary carbonitrides worsen the mechanical properties and oxidation behaviour of the subsurface layers and contribute to abrasion of the counterface. As far as the compound layer is concerned, the effect of thickness and compactness is at least as significant, if not more so, than that of composition. The results demonstrate the importance of designing the microstructure of nitrided layers to maximise the wear resistance of the treated components.

Corrosion resistance of HVOF WC–Co and TiC/Ni–Ti coatings sprayed on commercial steel

Abstract: Corrosion, erosion, and abrasion damage arefrequently found in equipment used in service in the oil and gas industries. This represents a challenge to materials engineers because of the deterioration of equipment and increased maintenance costs. Much work is being done on the evolution of corrosion of different types of coatings. The aim of the present work is to study the corrosion behaviour of two different coatings sprayed using high velocity oxyfuel technology. A commercial steel was coated with a CDS 1927 (WC–12Co) or TiC/Ni–Ti powder using a Plasma-Technik CDS–100 thermal spray gun. The powder was sprayed on to a cylindrical specimen 25·4 mm in diameter and 25·4 mm high. The corrosion potential v. time and potentiodynamic polarisation were studied inan ASTM standard cell with three electrodes. Air saturated seawater was used as the electrolyte.

Thermal spraying of functionally graded coatings for biomedical applications

Abstract: Biomedical requirements in a prosthesis are often complex and diverse in nature. These multifaceted requirements can be met by functionally graded materials (FGMs). Biomaterials for implants have to display high adaptability in order to suit the various stages of the biointegration of foreign material into the human body. The present paper analyses the preparation of functionally graded bioceramic coatings composed of calcium phosphate compounds. Two FGM systems were investigated. System 1 utilised the thermal response of different size ranges ofhydroxyapatite (HA) powders in the plasma flame to form gradients in biological and physical properties while system 2 combined two types of calcium phosphates, namely α tricalcium phosphate and HA, to form the FGM. The coating was graded according to adhesive strength and bioresorbability. The bond coat on the metal stub was deposited with a range of HA particles that provides high adhesive strength but has poor biocompatibility.

Oxygen thermochemical treatment combined with DLC coating for enhanced loadbearing capacity of Ti-6Al-4V

Abstract: In the present investigation, a novel duplex treatment of Ti–6Al–4V combining oxygen diffusion (OD) treatment with a carbon rich coating has been developed. A diamond like carbon (DLC) coating containing a small amount of titanium was deposited on to 0D treated Ti–6Al–4V using an unbalanced magnetron sputtering system. A graded intermediate layer of Ti/TiN /TiCN /TiC between the 0 D treated zone and the DLC coating was applied to facilitate good bonding. The loadbearing capacities of the duplex treated, as well as solely DLC coated, Ti–6Al–4V specimens were evaluated, both statically and dynamically using a friction monitored scratch tester, a ball on disc tribometer, and an indentation tester. It was found that both the staticand dynamic loadbearing capacities were significantly increased by a factor of 2·5 to 5 following the twofold treatment. Based onfurther fractographic examinations by high resolution SEM and finite element analysis, the excellent performance of the duplex system is discussed...

Advantages for automotive industry of plasma spray coating of Ai–Si cast alloy cylinder bores

Abstract: In the automotive industry the need for lower manufacturing costs, the use of fewer strategic materials, andthe need for easier, faster, and more environmentally sound coating processes is growing. This has led to the examination of a plasma powder spray process used in the application of coatings for surface modification. The present paper examines the use of the plasma powder spray process in coating aluminium-silicon cylinder block bores using a rotating plasma gun capable of producing coatings of reliable microstructure and integrity. Properties and microstructures of the applied coatings are presented and test results show that the necessary bond strength of the coating can be achieved without the use of a bond coat. Surface preparation before coating and surface finishing methods after coating are also discussed. Experience in Europe, Japan, and the USA shows that the plasma powder spray process offers a performance proven and cost effective solution for the coating of cylinder bores.

Application of ball cratering method to study abrasive wear

Abstract: The present paper discusses the feasibility of a new use of the ball cratering technique (Calotest) for studying the abrasive wear resistance of hard coatings. The few modifications that were necessary to make the Calotestequipment suitable for this task mainly concerned the calibration of rotational velocity and the controlled delivery of the abrasive–lubricant mixture. Three different types of material were tested to show the capability of the proposed method. These materials were: mono crystalline Si (100) wafers, plain carbon steel, and electroless Ni–P coatings on plain carbon steel in both the as deposited and heat treated conditions. The reproducibility of the values of the wear constants determined by means of the formulationdescribed in the literature prove that the ball cratering test is reliable for the determination of the abrasive wear behaviour of any of thecoatings studied.

Tempering behaviour of TiB2 coatings

Abstract: The compound TiB2 has been proposed as a wear resistant coating for tools because of its high hardness, but for practical applications it is also necessary to know its thermal and oxidation behaviour. For this reason in the present investigation TiB2 coatings were magnetron sputtered on to two different steel substrates and annealed in vacuum or air up to 800°C. After deposition the TiB2 coatings were nanocrystalline with a crystallite size ranging from 15 to 30 nm. The coatings deposited at low bias voltage showed a strong [001] fibre texture. With increasing bias the grain size decreased and the strong texture was lost. These structural modifications influenced the layer hardness, which decreased from about 30 to about 25 GPa at - 200 V bias, even though these lower hardness layers had higher residual compressive stresses. The oxidation of TiB2 started at 600°C. At this temperature the formation of TiOx was detectable. Moreover, the coating strength was overcome and a characteristic crack patter...

Deposition of synthetic diamond films: PVD., CVD., and multiplexed lasers

Abstract: Diamond has so many exceptional characteristics (high thermal conductivity, chemical inertness, high hardness, excellent insulation, lowest compressibility of any known material, semiconducting properties when doped) that industrial applications would be numerous if natural diamond was not such a high value commodity. If synthetic diamond (rather than diamondlike carbon) could be readily produced it would therefore have an enormous potential worldwide market. Variants on the chemical vapour deposition process are some of the most promising techniques for synthesising diamond, although they suffer from a number of key drawbacks. A more recent development is a novel multiplexed laser deposition system which may offer solutions to many of these problems; work is currently under way to gain a better understanding of this new technique.

Corrosion behaviour of TiN and CrN coatings produced by cathodic arc PVD process on mild steel substrate

Abstract: Hard coatings are often used for their mechanical properties such as wear and friction resistance. Corrosion behaviour may also be an important criterion for industrial applications of such coatings. There is abundant literature on both mechanical and corrosion properties of titanium nitride coatings, however there is much less published data on the properties of CrN coatings. In the present paper, the protective behaviour of TiN and CrN coatings on mild steel immersed in 3%NaCl solution was investigated. Various electrochemical techniques were used to characterise the behaviour of the coatings and the galvanic corrosion of the uncoated metals and coatings was thoroughly studied. The overall corrosion behaviour of the coatedsteel depends not only on the intrinsic electrochemical properties of the coatings, but also on surface defects such as roughness, droplets deposited during the PVD process, and pinholes, cracks, and scratches. In saline solution, the TiN coating has a poor protective effect co...

Cavitation erosion of steels processed with laser beam and optical discharge plasma

Abstract: The response of a solid state body to cavitation attack depends on impingement parameters as well as the properties of the eroded material. Changes to steel surface structural features intended to give cavitation resistance were induced by laser methods in order to test the usefulness of laser beam and optical discharge plasma surface remelting in protecting a 0·45% carbon steel and a 13% chromium steel against cavitation damage. Some light is also shed on the correlation between the performance of hardened layers during the incubation period of cavitation erosion and their physical properties.

Wear of plasma nitrided high speed steel

Abstract: In the present work the effects of different plasma nitriding processes on the high speed steel AISI M2 were investigated, comparing the wear performance and the microstructure of the steel. Specimens were nitrided at temperatures between 450 and 550°C for between 18 min and 2 h using a gas composition of 5 or 25%N2, balance H2. The wear test was carried out using an Amsler machine and a wheel-fiat type test. X-ray diffraction analyses, chemical composition depth profiles by glow discharge optical spectroscopy, microhardness tests, and optical and scanning electron microscopy of the layers were performed. The specimens nitrided at 550°C showed spalling in wear tests indicating a brittle surface, while the specimens nitrided for 1 h at 450 and 500°C (25%N2) demonstrated reduced friction coefficients and adhesive wear.

Effect of local thermal shock load on plasma sprayed thermal barrier coatings

Abstract: The number of cycles to failure of plasma sprayed thermal barrier coatings (TBCs) during thermal shockexperiments gives an indication of their performance in a real environment. The authors have subjected 0·3 mm thick ZrO2–8Y2O3 TBCs to a local thermal shock load with maximum surface temperatureof 1300 and 1400°C. The diameter of the flame was about 15 mm. During cycling, the temperature drop through the specimen thickness and the temperatureprofile of the back of the substrate were measured. The number of cycles to failure and the type of failure appeared to be dependent on the maximum surface temperature of the coating. After failure, a phase analysis was performed. In addition, the residual stress profile of the top coat was determined after 200 cycles with Tsurf = 1300°C. It is concluded that at 1300°C bond coat oxidation is the main cause of failure, while at 1400°C top coatdegradation leads to failure.

Influence of shot peening on superficial yield strength of spring steel in hard state

Abstract: The yield strength of a metallic technological surface with biaxial residual stresses may be determined using X-ray stress analysis techniques. Shot peening can increase the superficial proof stresses of a spring steel treated in the hard state, even though there is a considerable decrease in the half width of the X-ray diffraction profile. It can therefore be concluded that for a metallic material in the hard state, the half width parameter of the X -ray interference lines at the surface does not correctly reflect static strength.

Testing of zinc phosphate/stearate coating according to true conditions of contact in wire drawing

Abstract: Upsetting-sliding tests (USTs) were used to characterize the performance of a zinc phosphate/stearate coated 1035 steel under wire drawing conditions. Adjustments to the test were made in order to simulate contact pressures and effective plastic strains representative of the forming process. The mean Coulomb friction coefficient calculated from the UST analysis led to very good agreement between experimental and numerical results. It is shown that the damage mechanisms of the coating after drawing and after UST are similar.

Engineering The Subsurface of Borided Aisi H13 Steel

Abstract: Boriding has been employed with some success to increase the service life of commercial forging dies fabricated from the hot work steel AISI H13. This steel is poorly suited to the treatment, however, since an undesirable intermediate layer is formed between the hard boride coating and the core. In practice, a boride coating thickness well below that required for the application has been imposed by the need to minimise the thickness of this layer. An experimental study was undertaken with the aim of improving the effectiveness of a boriding treatment by engineering the subsurface of AISI H 13 steel. It was established that a carburising pretreatment markedly improved the microstructural features of the steel subsurface. The formation of the undesirable layer was completely inhibited within the range of boride thicknesses employed in commercial practice and severely limited at the higher thicknesses better suited to forging applications.

Thermal barrier coatings with two ZrO2 based ceramic layers

Abstract: A new thermal barrier coating (TBC) structure, composed of two ZrO2 based ceramic layers with different chemical composition on an MCrAlY bondcoating is proposed. In the double ceramic coatings, the lower layer consists of high strength ZrO2–Y2O3 and the upper layer of ZrO2–CeO2–Y2O3 with high heat shielding capacity. The new bilayer structure TBC was prepared by plasma spraying, and its properties, including thermal conductivity and thermal shock resistance, were evaluated. The thermal conductivity of the coatings showed good agreement with values calculated by the parallel plates model. The coatings also exhibited improved thermal shock resistance. This result is supported by FEM analysis, which revealed that tensile stress accompanying thermal shock is reduced and, in fact, applied only to the strong lower layer.

Effect of different compound layer and base material microstructures on microstrain and domain size of nitrided steel

Abstract: The inhomogenous microstrains in thermochemically treated materials can be described by calculating average crystallite size (dolnain size) limited by lattice defects and the root mean squared lattice strains in the domain using XRD analysis. When nitriding under different process parameters, specimens were removed from the experiment after different nitriding times in order to investigate the domain sizes and the mean squared lattice strains in the compound layer as a function of nitriding time. Studies of depth profiles from the compound layer were of special interest. The nitriding potential KN, a characteristic value for the composition of the nitriding atmosphere and the formation of nitride phases, was varied from 0·5 to 0·7 at temperatures of 540 and 575°C in order to form γ nitride. Because of the low penetration depth in ferritic materials, Cu Kα radiation was used to obtain information close to the surface layers only. To obtain depth profiles of the examined specimens, thin material lay...

Surface alloying of iron by laser melting: microstructure and mechanical properties

Abstract: Superficial laser induced melting produces non-equilibrium phases without changing the microstructure of the bulk and therefore alters specimen properties. Nitrided steels are of interest in a wide...

Surface modification of Ti–6Al–4V alloy by means of combined plasma nitriding and oxidising treatments

Abstract: The high reactivity that titanium and its alloys show with respect to nitrogen, oxygen, and carbon allows them to be used to improve the poor wear resistance of these materials. Nitriding and oxidising are two of the most widely used techniques for surface alloying with nitrogen and oxygen, as they produce modified surface layers characterised by improved hardness and wear resistance. A study of contemporary glow discharge nitriding and oxidising treatments was carried out with the purpose of evaluating the effect of oxygen introduced into the atmosphere of the nitriding process. Oxynitriding treatments were performed on Ti–6Al–4 V alloy specimens in atmospheres with variable contents of nitrogen and oxygen (0–21 vol.-%02) at four different temperatures (973, 1073, 1123, and 1173 K). Treated specimens showed a modified surface layer, constituted by an outer compound layer and an innerdiffusion layer: the characteristics of these layers depend on treatment temperature and gas composition. The comp...

Novel powertrain applications of thermal spray coatings

Abstract: Recent innovations in equipment, processes, and diagnostics make thermal spray coatings both practical and desirable for use in large scale automotive manufacturing.The advent of rotating devices and surface preparation methodologies such as high pressure water jet, electrodischarge machining, and chemical fluxes permit thermal spray coating applications on cylinder bore and valve seat areas of engines. Low carbon steel alloy composite and aluminium bronze coatings have been successfully tested on cylinder bores. The Fe–Fe3O4/Ni–NiO–CrO composite coating obtained by spraying carbon steel and nickel base alloy wires using a two wire arc gun works well as a valve seat material.

Variation of friction coefficient with percentage of metallic matrix in WC-Co coatings sprayed by HVOF

Abstract: Hardmetal (WC–Co) coatings are widely used in industry for their good resistance to wear. The continuous improvement of these materials has producedvarious commercial coatings of different metallic matrix content. In the present work, the factors that affect the friction coefficient of HVOF sprayed coatings of WC–12Co and WC-17Co have been evaluated, as have the wear mechanisms taking place during wear testing. By modifying the spraying distance, the two powders were used to produce different coatings in order to study the changes produced on the friction coefficient. A ball on disc test was used to evaluate the specimens (ASTM Standard G99–90) and the resultant structures were observed using optical and scanning electron microscopy. Energy dispersive spectrometrywas used to analyse the wear products generated during testing. The WC–17 Co coatings exhibited higher friction coefficients than WC–12Co coatings. This behaviour was attributed to the larger amount of metallic matrix present. The wear m...

Wear testing of hard coatings : More than wear rate ?

Abstract: The present paper describes how in depth analysis and modelling of wear data obtained from uni- and bidirectional wear tests were used to determine criteria for the selection of materials for tribological applications. Different wear modes in unidirectional ball on disc tests were linked to a load to roughness dependence between body and counterbody. The displacement of the ball is described using a particle flux model and the wear rate in bidirectional fretting tests is discussed, including a new concept based on dissipated frictional energy. The merits of the various approaches used to measure wear in tribological systems are also discussed.

Microstructural stability of thermal shayed WC–Co composite coatings in oxidising atmospheres at 450°C

Abstract: The present work stems from a development programme considering the use of WC–Co thermal spraycoatings in galvanising applications. In such applications, service conditions include exposure to air at 450°C. It is generally accepted that WC–Co composites do not oxidise significantly in air at temperatures below 500°C. However, the present results show significant oxidation of such coatingsat 450°C in air and in contact with a molten galvanising alloy. There appear to be two simultaneous oxidation reactions occurring, with WO3 forming from the tungsten carbide and tungsten phases and CoWO4 derived from the β-Co and cobalt containing carbide phases.

Process control of laser surface alloying

Abstract: In spite of the many advantages of laser surface treatment, such as high production rates and low induced thermal distortion, and its great potential for modifying the surface properties of a wide range of new and existing materials, industrial applications are still limited. This is not only because of the relatively high cost of laser systems; another significant problem is the high sensitivity of laser surface treatments to process disturbances and to small changes in processing parameters. A change of only 100/0 in absorbed laser power, for example, can cause a change of up to 50% in case depth. Real time process control is therefore required to increase the stability, reproducibility, efficiency, and productivity of laser surface treatment