Journal ArticleDOI

# Morphology and microstructure evolution of Ti-50 at.% Al cathodes during cathodic arc deposition of Ti-Al-N coatings

29 Jun 2017-Journal of Applied Physics (AIP Publishing LLC)-Vol. 121, Iss: 24, pp 245309

AbstractToday's research on the cathodic arc deposition technique and coatings therefrom primarily focuses on the effects of, e.g., nitrogen partial pressure, growth temperature, and substrate bias. Detailed studies on the morphology and structure of the starting material—the cathode—during film growth and its influence on coating properties at different process conditions are rare. This work aims to study the evolution of the converted layer, its morphology, and microstructure, as a function of the cathode material grain size during deposition of Ti-Al-N coatings. The coatings were reactively grown in pure N2 discharges from powder metallurgically manufactured Ti-50 at.% Al cathodes with grain size distribution averages close to 1800, 100, 50, and 10 μm, respectively, and characterized with respect to microstructure, composition, and mechanical properties. The results indicate that for the cathode of 1800 μm grain size the disparity in the work function among parent phases plays a dominant role in the pronounced...

Topics: , Microstructure (55%), Grain size (54%), Layer (electronics) (51%), Coating (50%)

### Introduction

• Morphology and microstructure evolution of Ti-50 at.%.
• This work aims to study the evolution of the converted layer, its morphology, and microstructure, as a function of the cathode material grain size during deposition of Ti-Al-N coatings.
• Detailed understanding and abundant research material on Ti-Al-N,4 ZrAl-N,5 Ti-Si-N,6 etc., are readily available.
• In-depth knowledge of such mechanisms is needed for a comprehensive understanding of the arc deposition process and perhaps a mean to control the microstructure and composition of arc deposited coatings.
• In addition, the effect of cathode material grain size on coating microstructure, composition, and mechanical properties is discussed.

### II. EXPERIMENTAL DETAILS

• Classified by different average grain sizes distributions, four grades of titanium–aluminum with equal atomic percentage were used in this study.
• This ensures that the arc traverses the surface of the piece only once (single trigger event).
• To capture the surface state after multiple arcing events (steady state arcing) and to analyze differences in the resulting coatings, full sized circular cathodes having a diameter of 63 mm were also made from all four grades.
• The macro-particle areal density of the coatings was measured by the box-counting method, i.e., by dividing SEM micrograph with an area of 2015 lm2 into 60 boxes and then counting the macro-particles in each box.
• The load of 25 mN and taper polishing were used to mitigate the influences from the substrate and surface roughness on the recorded hardness.

### III. RESULTS

• Figure 2 shows SEM micrographs of polished virgin materials prepared from four grades.
• These diffractograms show that during the initial stage of arcing the 1800 lm-grade retains its virgin composition of Ti and Al, the 100 lm-grade also shows the existence of just virgin phases, the 50 lm-grade shows the additional trace of f (Al5Ti2), and the 10 lm-grade additionally contains the intermetallic phase g (Al2Ti).
• The converted layer on the 1800 lm-grade hosts craters with diameters ranging from sub-micron to 3100 lm.
• Cross-sections of the converted layer on the four grades are shown in Fig.
• The coatings grown by the 100 lm-grade, as well as 50 lm-grade, have only spherical macro-particles, and the coating grown by the 100 lm-grade shows the fewest macro-particles among the four coatings.

### A. Formation and evolution of the converted layers

• The composite manufactured material of all the four grades of Ti-50 at.%.
• A third factor yielding a more pronounced erosion of Al than Ti is related to the fact that metals with low melting temperatures emanate more macro-particles.
• It anchors the arc at this position and brings the liquid Al to boil.
• The grain size is too small for arc anchoring in the other three grades, which explains why macro-bubbles are not observed for these grades.
• On the other hand, the large interface area and poor thermal conductivities in the cases of 50 and 10 lm-grades generate more favorable conditions for the formation of intermetallic phases.

### B. Steady-state condition of the converted layers and the effect on coatings

• The multiple instances of the cathode spot even out the macro-bubbles observed during the initial stage of the converted layer on the 1800 lm-grade [see Fig. 7(i)].
• Al has a lower melting temperature than Ti, it is expected that these flattened macro-particles are Al rich and they require a lower temperature or a longer time to solidify; as a result, these macro-particles reach the substrate in the liquid state.
• The relatively thick (50 lm) converted layer on the surface of 1800 lm-grade is a result of preferential erosion and the anchoring of the cathode spot.
• The temperature is expected to be quite high due to the residual heat generated from the self-sustaining reaction, and the ignition of the cathode spot further increases the temperature of the region surrounding this site.
• The high macro-particle density of the coatings grown by 50 lm-grades compared to 100 lm-grade is related to the self-sustaining exothermic reaction, which consumes the Al phase.

### V. CONCLUSION

• Al powder metallurgical cathodes, as a function of their grain sizes (1800, 100, 50 and 10 lm), during the cathodic arc deposition of Ti-Al-N coatings.
• These two factors contribute to the high Al content in the deposited coatings.
• The reduction in grain size to 100 lm enhances the intermixing of the parent phases, which mitigates the anchoring phenomenon and results in the coatings having equal content of Ti and Al.
• Further reduction in the grain size creates favorable conditions for the initiation of the self-sustaining reaction to form the c phase.
• For the 10 and 50 lm grain size cathodes the self-sustaining reaction is the dominant factor affecting the evolution of the morphology and microstructure of the cathodes during arcing.

Did you find this useful? Give us your feedback

Content maybe subject to copyright    Report

Morphology and microstructure evolution of Ti-50 at.% Al cathodes during cathodic arc
deposition of Ti-Al-N coatings
Bilal Syed, Jianqiang Zhu, Peter Polcik, Szilard Kolozsvari, Greger Håkansson, Lars Johnson, Mats Ahlgren,
Mats Jöesaar, and Magnus Odén
Citation: Journal of Applied Physics 121, 245309 (2017); doi: 10.1063/1.4990425
View online: http://dx.doi.org/10.1063/1.4990425

Morphology and microstructure evolution of Ti-50 at.% Al cathodes
during cathodic arc deposition of Ti-Al-N coatings
Bilal Syed,
1
Jianqiang Zhu,
1
Peter Polcik,
2
Szilard Kolozsvari,
2
Greger Ha˚kansson,
3
Lars Johnson,
4
Mats Ahlgren,
4
Mats J
oesaar,
1,5
and Magnus Od
en
1
1
Nanostructured Materials, Department of Physics, Chemistry, and Biology (IFM), Link
oping University,
oping, Sweden
2
PLANSEE Composite Materials GmbH, DE-86983 Lechbruck am See, Germany
3
Ionbond Sweden AB, Box 1161, SE-58111 Link
oping, Sweden
4
Sandvik Coromant, 126 80 Stockholm, Sweden
5
SECO Tools AB, SE-737 82 Fagersta, Sweden
(Received 3 May 2017; accepted 14 June 2017; published online 29 June 2017)
Today’s research on the cathodic arc deposition technique and coatings therefrom primarily
focuses on the effects of, e.g., nitrogen partial pressure, growth temperature, and substrate bias.
Detailed studies on the morphology and structure of the starting material—the cathode—during
ﬁlm growth and its inﬂuence on coating properties at different process conditions are rare. This
work aims to study the evolution of the converted layer, its morphology, and microstructure, as a
function of the cathode material grain size during deposition of Ti-Al-N coatings. The coatings
were reactively grown in pure N
2
discharges from powder metallurgically manufactured Ti-50 at.%
Al cathodes with grain size distribution averages close to 1800, 100, 50, and 10 lm, respectively,
and characterized with respect to microstructure, composition, and mechanical properties. The
results indicate that for the cathode of 1800 lm grain size the disparity in the work function among
parent phases plays a dominant role in the pronounced erosion of Al, which yields the coatings rich
in macro-particles and of high Al content. We further observed that a reduction in the grain size
of Ti-50 at.% Al cathodes to 10 lm provides favorable conditions for self-sustaining reactions
between Ti and Al phases upon arcing to form c phase. The combination of self-sustaining reaction
and the arc process not only result in the formation of hole-like and sub-hole features on the con-
verted layer but also generate coatings of high Al content and laden with macro-particles.
I. INTRODUCTION
Cathodic arc deposition is the quotidian PVD technique
used to deposit hard coatings on cutting tools. This technique
owes its expedience to the fact that the metal plasma formed
during the process is highly ionized and dense.
1
It begets
good adhesion, high deposition rates, and dense coatings
with compressive stress,
2
which result in superior mechani-
cal properties.
3
The demands for higher cutting speeds during turning
and milling operations require the coatings on cutting tools
to withstand extreme and harsh conditions of high tempera-
tures and pressures, which has motivated extensive research
on multinary ceramic materials. For example, detailed under-
standing and abundant research material on Ti-Al-N,
4
Zr-
Al-N,
5
Ti-Si-N,
6
etc., are readily available. Current research
in this ﬁeld has mainly been directed to tailor and understand
the optimum microstructure by compositional variance.
These multinary ceramic coatings are generally grown by
using powder metallurgically produced multiphase cathodes,
such as Ti-Al,
7
Zr-Al,
8
Ti-Si,
9
etc., arced in the presence of
the reactive gas N
2
.
The perpetual existence of cathode spot(s) on the cath-
ode surface is the very essence of the cathodic arc deposi-
tion process. In addition, the cathode spots dynamics can
be inﬂuenced by, e.g., the microstructure of the cathode,
which in turn may lead to changes in the microstructure of
the coatings and, hence, also of its properties.
9
Depending
on cathode material, the electron temperatures between 2
and 5 eV have been reported in the vicinity of cathode
spots.
10
The presence of such high electron temperatures,
in proximity to the cathode surface, modiﬁes its micro-
structure to a depth of several micrometers to form a con-
verted layer.
11
Most investigations on how to control coating micro-
structure and composition by manipulating the cathodic arc
process are chieﬂy focused on the aspects of ambient gas
pressure,
12
bias voltage,
13
deposition temperature,
14
and
magnetic steering
15
of the cathode spot. However, the inﬂu-
ence of the microstructure of the cathode on the synthesis of
coating, their microstructure, and composition is almost
never considered. Although the interplay of multiple phases
in the cathode, the reactive gas, and the localized ultrahigh
temperatures due to the transient nature of the cathode spots
causes intermixing of the parent phases and the formation of
new phases in the converted layer. In-depth knowledge of
such mechanisms is needed for a comprehensive understand-
ing of the arc deposition process and perhaps a mean to con-
trol the microstructure and composition of arc deposited
coatings. This work focuses on the evolution of the con-
verted layer on the cathode surface, its morphology, and
microstructure, as a function of the Ti-50 at.% Al cathode
0021-8979/2017/121(24)/245309/11/$30.00 Published by AIP Publishing.121, 245309-1 JOURNAL OF APPLIED PHYSICS 121, 245309 (2017) material, with the average grain size distribution ranging between 10 and 1800 lm, during the arc deposition of Ti- Al-N coatings. In addition, the effect of cathode material grain size on coating microstructure, composition, and mechanical properties is discussed. II. EXPERIMENTAL DETAILS Classiﬁed by different average grain sizes distributions, four grades of titanium–aluminum with equal atomic per- centage were used in this study. The average grain sizes were 1800, 100, 50, and 10 lm and they were labeled as 1800 lm-grade, 100 lm-grade, 50 lm-grade, and 10 lm- grade, respectively. Monosized powder mixtures of Ti and Al were prepared by sieving to the right grain size and used as starting material during cathode manufacturing. All four grades were fabricated in the same manner, i.e., densiﬁcation of the powder mixture by pressing and forging followed by solid state sintering at 400 C. This fabrication route resulted in a solid material with a density greater than 99.7% of theo- retical density and below 300 lg/g of oxygen content. In order to study the arc events on the virgin material, pieces with dimensions 1.5 1.5 0.5 cm 3 were cut by a diamond blade from all four grades. These pieces were then polished to a mirror like ﬁnish by following the series of grinding, lapping, and polishing steps. The pieces were then cleaned in an ultrasonic ethanol bath. Each piece was fas- tened with electrically conducting silver containing glue on the surface of a dummy cathode [shown in Fig. 1(a)]. The triggering event serves a dual purpose, one being the initia- tion of the arc and the other being that the mechanical force imparted by the trigger on the piece breaks free the estab- lished electrical contact between the piece and the dummy cathode. This ensures that the arc traverses the surface of the piece only once (single trigger event). An industrial arc chamber (Metaplas MZR323) was used for all the experi- ments. The parameters used during the arcing of the pieces were 2.0 10 6 Pa base pressure, 4.5 Pa nitrogen pressure, and 80 A arc current. To capture the surface state after multiple arcing events (steady state arcing) and to analyze differences in the result- ing coatings, full sized circular cathodes having a diameter of 63 mm were also made from all four grades. These cathodes were arced in N 2 (reactive) ambience with the parameters of 2.0 10 6 Pa base pressure, 4.5 Pa N 2 pres- sure, 30 V bias voltage, 75 A arc current, 300 C process temperature, and a constant evaporation time is achieved by a predetermined setpoint of 300 Ah. Coatings were deposited on polished and ultrasonically cleaned WC-13 wt. % Co sub- strates (ISO SNUN120408, 1.2 1.2 0.5 cm 3 ) mounted on a drum ﬁxture rotating with a speed of 3 rpm as shown in Fig. 1(b). Seven substrates were mounted such that they cov- ered an angular range of 0 to 49 from the surface normal of the cathode. Prior to the deposition of coatings, the sub- strates were etched with Ar ions. The microstructures of virgin and arced material, as well as the coatings, were investigated using a LEO 1550 SEM equipped with an Oxford energy dispersive x-ray spectrome- ter (EDX). An acceleration voltage of 10 kV at a working distance of 5 mm was used for imaging, while an acceleration voltage of 20 kV at a working distance of 8.5 mm was used for elemental mapping and compositional analysis. A TiAlN sample of known composition was used to obtain calibrated EDX data, resulting in an accuracy of the compositional analysis of 5 at.%. For viewing the microstructure of the converted layers by SEM, cross-sectional samples having dimensions of 1 0.5 0.5 cm 3 were cut by a diamond blade. These samples were then embedded in a carbon based resin and polished by using the same regimen mentioned earlier. A Zeiss 1540 EsB CrossBeam (FIB) was used to section the cathode surface features to reveal their microstructure. The macro-particle areal density of the coatings was mea- sured by the box-counting method, i.e., by dividing SEM micrograph with an area of 2015 lm 2 into 60 boxes and then counting the macro-particles in each box. The number of macro-particles counted from 60 boxes was then divided by the area of micrograph to calculate the areal density of macro-particles. The lower limit on the radii of countable macro-particles was set at 50 nm. Four micrographs from dif- ferent locations along the diagonal of each coated substrate were used to gain a reliable average macro-particle areal density. X-ray diffractometry of arced pieces, arced cathodes, and coatings was performed in 1 grazing incidence geome- try 16 using a PANalytical Empyrean diffractometer and Cu-Ka radiation. Young’s modulus of 429 GPa and FIG. 1. (a) Schematic drawing of how a piece of Ti-50 at.% Al is glued to the surfaces of a dummy cathode. (b) Schematic drawing of the deposition geometry showing how the cathode, drum ﬁxture, and the substrates were positioned (not to scale). 245309-2 Syed et al. J. Appl. Phys. 121, 245309 (2017) Poisson’s ratio of 0.19, as indicated by Mittemeijer et al. for TiN 0.98 , 17 were used to measure the stress of the coatings by using the sin 2 w method. 16 The hardness of the coatings was evaluated by using a diamond Berkovich tip mounted on a load-controlled UMIS nanoindenter. The area function of the tip was calibrated using a fused silica reference sample. The hardness measure- ments were obtained from load-displacement curves by employing the method developed by Oliver and Phar. 18 The hardness values reported were averaged from 40 indents at 25 mN load with a maximum achieved penetration depth of 0.18 lm on tapered (tapering angle 5 ) 19 and polished cross section of the coatings. The load of 25 mN and taper polish- ing were used to mitigate the inﬂuences from the substrate and surface roughness on the recorded hardness. III. RESULTS Figure 2 shows SEM micrographs of polished virgin materials prepared from four grades. The grain size evalua- tion by the tangent rule performed on theses micrographs is in agreement with the result of the sieve analysis, suggesting limited or no grain growth during manufacturing. Figure 3 shows the x-ray diffractograms of polished virgin materials of all the four grades, showing that the virgin material of all four grades has a duplex structure of two phases: Al and Ti. The traces of the arc on polished pieces of four different grades are shown in Fig. 4. In general, Al grains suffer more erosion than Ti grains, and the largest craters tend to appear at the shared peripheries of Ti and Al grains. The higher erosion rate of Al is expected since Al compared to Ti has a lower work function as well as cohesive energy. The reported work functions of Ti and Al ranges from 4.33 to 4.53 eV (Refs. 1, 20,and21) and 4.08 to 4.28 eV, 1,20,22 respectively. In the case of Ti-Si, 9 Zhu et al. has shown that the phase with a low work function experiences a higher fre- quency of cathode spot events. The reported cohesive energies of Ti and Al are 4.86 eV/atom (Ref. 23) and 3.34 eV/atom, 23 respectively. It has been established that, in general, cohesive energies of metals have an inverse relationship with the ion erosion rate. 24 On the 1800 lm-grade, the largest crater appears on an Al grain and in close proximity of a Ti grain, and the diame- ter of this crater is around 850 lm, while the largest crater on a Ti grain is around 43 lm. An additional feature appears on this grade in the form of macro-bubbles (solidiﬁed bubbles) of Al located at the interface between the Ti and Al grains, as shown by the overlaid EDX elemental map in Fig. 5(a). Figure 5(b) shows the FIB excavated cross-section of the macro-bubble, conﬁrming that these features are hollow. For the 100 lm-grade, the recorded diameter of the larg- est craters on Al and Ti grains are approximately 18 and 2 lm, respectively. For the 50 lm-grade, the channels of Al sandwiched between Ti grains suffer higher erosion, the cra- ters left at this channel site encompass the Al channel as well as the edges of the Ti grains. The largest crater of such kind has an approximate diameter of 27 lm, while the largest cra- ter left on the Al and Ti grains have diameters of around 5 lm and 2.5 lm, respectively. FIG. 2. SEM micrograph of virgin material (a) 1800 lm-grade, (b) 100 lm- grade, (c) 50 lm-grade, and (d) 10 lm- grade. FIG. 3. X-ray diffractograms of all four grades of virgin material. 245309-3 Syed et al. J. Appl. Phys. 121, 245309 (2017) The erosion on the 10 lm-grade appears to be more pro- nounced compared to both the 100 and 50 lm-grades. The largest observable crater on the 10 lm-grade has an approxi- mate diameter of 35 lm. For the 10 lm-grade, the virgin material [see Fig. 2(d)] consists of Ti grains with a diameter smaller than 5 lm dispersed in the Al matrix. There appear to be abundant craters of diameters greater than 5 lm. This widespread dispersion of small Ti grains and abundance of craters larger than Ti grains makes it impossible to ascertain the diameter of largest craters on the Ti and Al grains. The phase compositions of all the pieces after being arced with a single trigger event are shown in the x-ray diffractro- grams in Fig. 6. These diffractograms show that during the initial stage of arcing the 1800 lm-grade retains its virgin composition of Ti and Al, the 100 lm-grade also shows the existence of just virgin phases, the 50 lm-grade shows the additional trace of f (Al 5 Ti 2 ), and the 10 lm-grade addition- ally contains the intermetallic phase g (Al 2 Ti). Although the ambient gas during the single trigger event was N 2 ,no N-containing compounds were detected. The optical image in Figs. 7(a)–7(d) shows the virgin and worn cathodes of all the four grades. The roughness of arced cathodes can be seen to decrease from the 1800 lm- grade to 50 lm-grade and then increase for the 10 lm-grade. SEM micrographs of the topography of the converted layers are shown in Figs. 7(i)–7(iv). The converted layer on the 1800 lm-grade hosts craters with diameters ranging from sub-micron to 3100 lm. This converted layer is enriched with craters of 2000 lm diameter. The crater diameters on the converted layer of 100 lm-grade range from sub-micron to 62 lm. The converted layer on the 50 lm-grade hosts cra- ters as well as hole-like features. The diameters of the craters range from sub-micron to 70 lm. The average diameter of the hole-like features in the converted layer of the 50 lm- grade is 25 lm, and these hole-like features occasionally have sub-holes. The diameters of the craters on the converted layer of the 10 lm-grade range from sub-micron to 22 lm and the frequently occurring hole-like features are larger FIG. 4. SEM micrograph showing arc traces from a single trigger event on (a) polished 1800 lm-grade, (b) pol- ished 100 lm-grade with an inset show- ing a magniﬁed micrograph of Ti grain, (c) polished 50 lm-grade (* denote the regions of arced Al sandwiched between Ti grains), and (d) polished 10 lm-grade. FIG. 5. (a) Scanning electron micro- graph of macro-bubble with an EDX elemental map. (b) The excavation of macro-bubble achieved by Fib. FIG. 6. X-ray diffractograms of all four grades after a single trigger event. 245309-4 Syed et al. J. Appl. Phys. 121, 245309 (2017) ##### Citations More filters Journal ArticleDOI Abstract: Author(s): Zohrer, S; Golizadeh, M; Koutna, N; Holec, D; Anders, A; Franz, R | Abstract: Many properties of cathodic arcs from single-element cathodes show a correlation to the cohesive energy of the cathode material. For example, the burning voltage, the erosion rate, or, to a lesser extent, plasma properties like electron temperatures or average ion energy and charge states. For multi-element cathodes, various phases with different cohesive energies can initially be present in the cathode, or form due to arc exposure, complicating the evaluation of such correlations. To test the influence of morphology and phase composition of multi-element cathodes on cathodic arc properties, a Nb-Al cathode model system was used that includes: pure Nb and Al cathodes; intermetallic Nb3Al, Nb2Al and NbAl3 cathodes; and three composite Nb-Al cathodes with atomic ratios corresponding to the stoichiometric ratios of the intermetallic phases. Pulsed cathodic arc plasmas from these cathodes were examined using a mass-per-charge and energy-per-charge analyzer, showing that charge-state-resolved ion energy distributions of plasmas from the intermetallic and corresponding composite cathodes are nearly identical. An examination of converted layers of eroded cathodes using x-ray diffraction and scanning electron microscopy indicates the formation of a surface layer with similar phase composition for intermetallic and their corresponding composite cathode types. The average arc voltages do not follow the trend of cohesive energies of Nb, Al and intermetallic Nb-Al phases, which have been calculated using density functional theory. Possible reasons for this effect are discussed based on the current knowledge of multi-element arc cathodes and their arc plasma available in literature. 6 citations Journal ArticleDOI Abstract: Nowadays, multi-element cathodes are frequently employed to grow multi-element thin films and coatings using cathodic arc deposition processes. During cathode erosion, the cathode spot sequentially ignites on the cathode surface and imposes melting–solidification cycles that lead to material intermixing and the formation of a modified layer on the cathode surface. To allow us to study these surface modifications, a 10 μm thick Mo/Al multilayer coating was sputter-deposited onto a standard Ti arc cathode. This cathode was eroded by a dc steered arc discharge for a short duration enabling the observation of single craters formed by type 1 and 2 cathode spots. Furthermore, separated clusters of overlapping craters and a fully eroded surface caused by different stages of erosion were differentiated when scanning the erosion track in the lateral direction. Cross sections of single craters were prepared by focused ion beam techniques while metallographic methods were applied to obtain cross sections of overlapping craters and the modified layer. The layers of the multilayer coating acted as trace markers providing new insights into the material intermixing within craters, the material displacements during crater formation, the plasma pressure acting on the craters, and the temperature gradient (heat-affected zone) below the craters. The observations are discussed within the framework of established arc crater formation models. 5 citations Journal ArticleDOI Abstract: The differences in work function (W.F.) and cohesive energy (C.E.) of the phases constituting the cathode, plays an important role in the formation of the converted layer at its near-surface region during cathodic arc deposition. As a consequence, this also affects the deposition conditions for the coatings. In this study, we explore the effect of W.F. and C.E. of the constituent phases during arc evaporation by utilizing two kinds of customized Ti-50 at.% Al cathodes with different phase compositions. Our results show that during reactive arc evaporation the disparity in W.F. and C.E. among the constituent phases of Ti-50 at.% Al cathodes leads to preferential erosion of the phases with lower W.F. and C.E. The aforementioned preferential erosion begets higher surface roughness on the Ti-50 at.% Al cathode with a wider range of W.F. and C.E. disparity. It is also observed that the thermal conductivity of the Ti-50 at.% Al cathode plays a dominant role in the deposition rate of Ti-Al-N coating. This article also presents how the surface geometry of the cathode in the presence of arc guiding magnetic field significantly influences the microstructure of the deposited coatings. 5 citations Journal ArticleDOI , Hao Kang1 Abstract: Grain size, growth morphology, and grain boundary were considered to affect hardness and fracture toughness of hard coatings. An effective approach is currently developed to improve their hardness and toughness via tailoring their architectures through grain boundary control. V-Al-C coatings consisting of variable architectures were prepared by reactive sputtering V2AlC target with different CH4 flow rates in this study. The results indicated that, when the carbon content increased from 37.32 at.% to 71.4 at.%, the microstructure of V-Al-C coatings was tailored to span a wide changes from coarse columnar grain to fibrous columnar grains and finally to nanocomposite structure consisted of the (V, Al)C nanocrystallites and sp2-rich a-C. Especially, the maximum hardness of 28.74 GPa and the excellent toughness with high H/E value of 0.11 were obtained once the coating displayed fibrous columnar structure, such good mechanical properties benefited the lowest wear rate of 2.8 × 10−16 m3/Nm. Instead, the nanocomposite structure showed the lower fracture resistance due to the dominated amorphous carbon matrix. All coatings with nanocomposite structure exhibited low friction coefficient of ∼0.18, which was attributed to the coupling lubrication originated from both V2O5 Magneli phases and amorphous carbon formed during friction process. The enhanced mechanical and tribological properties were also discussed in terms of the columnar boundaries evolution as a function of carbon content. 4 citations Journal ArticleDOI 2 citations ##### References More filters Journal ArticleDOI Abstract: The indentation load-displacement behavior of six materials tested with a Berkovich indenter has been carefully documented to establish an improved method for determining hardness and elastic modulus from indentation load-displacement data. The materials included fused silica, soda–lime glass, and single crystals of aluminum, tungsten, quartz, and sapphire. It is shown that the load–displacement curves during unloading in these materials are not linear, even in the initial stages, thereby suggesting that the flat punch approximation used so often in the analysis of unloading data is not entirely adequate. An analysis technique is presented that accounts for the curvature in the unloading data and provides a physically justifiable procedure for determining the depth which should be used in conjunction with the indenter shape function to establish the contact area at peak load. The hardnesses and elastic moduli of the six materials are computed using the analysis procedure and compared with values determined by independent means to assess the accuracy of the method. The results show that with good technique, moduli can be measured to within 5%. 20,195 citations Journal ArticleDOI TL;DR: In the data for the 63 elements, trends that occur simultaneously in both the columns and the rows of the periodic table are shown to be useful in predicting correct values and also for identifying questionable data. Abstract: A new compilation, based on a literature search for the period 1969–1976, is made of experimental data on the work function. For these 44 elements, preferred values are selected on the basis of valid experimental conditions. Older values, which are widely accepted, are given for 19 other elements on which there is no recent literature, and are so identified. In the data for the 63 elements, trends that occur simultaneously in both the columns and the rows of the periodic table are shown to be useful in predicting correct values and also for identifying questionable data. Several illustrative examples are given, including verifications of predictions published in 1950. 3,199 citations Journal ArticleDOI Abstract: Semiempirical interatomic potentials have been developed for Al,$\ensuremath{\alpha}\ensuremath{-}\mathrm{Ti},$and$\ensuremath{\gamma}\ensuremath{-}\mathrm{TiAl}$within the embedded atom method (EAM) formalism by fitting to a large database of experimental as well as ab initio data. The ab initio calculations were performed by the linearized augmented plane wave (LAPW) method within the density functional theory to obtain the equations of state for a number of crystal structures of the Ti-Al system. Some of the calculated LAPW energies were used for fitting the potentials while others for examining their quality. The potentials correctly predict the equilibrium crystal structures of the phases and accurately reproduce their basic lattice properties. The potentials are applied to calculate the energies of point defects, surfaces, and planar faults in the equilibrium structures. Unlike earlier EAM potentials for the Ti-Al system, the proposed potentials provide a reasonable description of the lattice thermal expansion, demonstrating their usefulness for molecular-dynamics and Monte Carlo simulations at high temperatures. The energy along the tetragonal deformation path (Bain transformation) in$\ensuremath{\gamma}\ensuremath{-}\mathrm{TiAl}$calculated with the EAM potential is in fairly good agreement with LAPW calculations. Equilibrium point defect concentrations in$\ensuremath{\gamma}\ensuremath{-}\mathrm{TiAl}$are studied using the EAM potential. It is found that antisite defects strongly dominate over vacancies at all compositions around stoichiometry, indicating that$\ensuremath{\gamma}\ensuremath{-}\mathrm{TiAl}\$ is an antisite disorder compound, in agreement with experimental data.

427 citations

Journal ArticleDOI
Abstract: A thermodynamic description of the Ti-Al system has been developed. Three different analytical descriptions were used to describe the three different types of phases occurring in the Ti-Al system: the stoichiometric compounds, the disordered solution phases, and the ordered inter-metallic compounds which have homogeneity ranges. A least-squares technique was used to optimize the thermodynamic quantities of the analytical description using experimental data available in the literature. The calculated phase diagram, as well as the thermodynamic func-tions, agree well with the critically evaluated experimental data from the literature.

330 citations

Journal ArticleDOI
Abstract: The composition of cathode mass loss was analysed for cadmium, copper and molybdenum vacuum arcs. It showed that two dominant flows are present, one consisting of ions, the other of molten droplets which have sizes in the order of microns to tens of microns. The droplet flow is mainly oriented along the cathode plane and is strongly dependent on the fusion temperature of the cathode metal and the charge transfer by the arc. The cathode mass loss in vapour form is considered to be small.

327 citations