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Growth behavior of vanadium carbide coatings on steel substrates by a salt bath immersion coating process
Thin Solid Films, 249 (1994) 54-61
54
Growth behavior of vanadium carbide coatings on steel substrates by a salt bath immersion coating process Tohru Arai and Sigeo Moriyama Toyota Central Research and Development Labs., Inc. 41-1, Yokomichi, Nagakute, Nagakute-cho, Aichi-Gun, Aichi-Ken (Japan) (Received July 20, 1993; accepted February 16, 1994)
Abstract Nucleation and growth behavior, micro-structure, and preferred orientation of vanadium carbide coatings formed using a borax bath were studied, focussing on the effects of coating temperature and substrate steel. The coatings were formed through three stages; nucleation and growth of relatively coarse grains, sub-micron sized grain formation on these, and growth into thick coatings by successive deposition of coarse grains. Preferred orientation was highly influenced by temperature and substrate steel.
1. Introduction In the thermo-reactive deposition and diffusion (TRD) carbide coating process [1] using a borax bath containing no carbon, carbide coatings grow onto carbon-containing substrates through the combination of carbide-forming-element atoms in the bath with carbon atoms exclusively supplied from the substrates. A number of papers have been published about this coating process to clarify various points, such as the reaction in the borax salt, the mechanism of carbide layer formation, factors controlling the growth rate of carbide layers, properties of carbide layers and carbide-coated materials [1-3]. For better understanding of the coating formation and properties of the coatings, however, the micro-structure of the coatings (the shapes and sizes of grains, preferred orientations, etc.) as a result of heterogeneous nucleation and growth processes must be examined. The purpose of this paper is, therefore, to show the results of experiments on nucleation and growth of vanadium carbide (VC) coatings onto steel substrates. Initial nucleation and growth at the coating-substrate interface has not been well understood in this coating process. There have been a number of research papers on the nucleation and growth during chemical vapour deposition (CVD) [4-9], and very few papers have dealt with this in the diffusion carbide coating by the powder pack method and the gas method [10-12]. The salt bath immersion process in ambient atmosphere has two advantages over CVD and the powder method in observing the very early stage of coating deposition. It is much easier to coat specimens within a very short immersion time if the specimens are sufficiently small to rapidly
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reach the bath temperature. The shortest time used in this research was 5 s, which was employed in very few of the papers on CVD [5]. The other advantage is the capability of rapid cooling from the coating temperature to quench the growth pattern. In contrast, if the salt agent remained on the specimen surfaces it may disturb surface observations. This can be solved by well-managed cleaning operation and careful observation.
2. Experimental details
2.1. Carbide coating procedure VC coating was carried out by the salt bath immersion process [1]. Polished small specimens of AISI 1045, W1 (0.95 wt.% C), D2, and M2 steels in the annealed state were immersed for the specified time in borax baths with added vanadium penta-oxide flakes and boron carbide powders or aluminum pieces as reducing agents for vanadium oxide, or in baths with ferro-vanadium powders, and then air cooled or quenched in water. Differing the cooling methods did not produce any serious effect on the results. A bath containing 20 wt.% vanadium penta-oxide and 5 wt.% boron carbide was mainly used. A bath with varied amounts of vanadium penta-oxide and boron carbide or aluminum, and a bath with varied amounts of ferro-vanadium were employed only for examining the effect of bath composition. The baths were kept at 1038-1298 K in ambient atmosphere. A very wide range of immersion times, 5-86.4 Ks, was adopted for W1 for the detailed examination of the initial stage of coating growth. An immersion time
~L 1994 - - Elsevier Science S.A. All rights reserved
T. Arai, S. Moriyama / Growth behaviour o f VC coatings on steel substrates
range of 300-86.4 Ks was employed for D2 to examine preferred orientation. 1045 and M2 were immersed only for 21.6 Ks for comparison with Wl and D2. Thin specimens, 1 mm thick and 8 mm in diameter, were used for the immersion times shorter than 3.6 Ks to ensure a rapid rise of specimen temperature. Specimens 15 mm long and 8 mm in diameter, were used for a longer immersion time so that the specimens contain sufficient carbon atoms for thicker coating formation. Temperature measurement using specimens with a welded thermocouple, having diameters of 0.64 mm, concluded that 1 mm thick specimens reached a bath temperature of 1188 K after about 15 s and 15 mm long specimens after about 180 s. It was roughly estimated that 1 mm thick specimens reached above 1073 K and 15 mm long specimens 573 K after 5 s immersion in a 1188 K bath. Scanning electron microscopy (SEM) observations of fractured cross-sections of W1 steel coated at 1183 and 1283 K showed that the morphology of the coating on 1 mm thick specimens by immersion for 1.8 Ks was in good accordance with those on 15 mm long specimens by immersions for 7.2, 14.4, and 72 Ks. Also, no remarkable discontinuity was found on the curves showing the change of preferred orientation with increased thickness. It is concluded from this evidence that the differences in specimen size and the resulting heating-up time did not affect the purpose of this research.
2.2. Observations The bath salt attached to the specimens was washed off by immersion in hot water, followed by ultrasonic cleaning in ethanol and acetone. SEM surface observation was carried out mainly on W1 specimens. Morphology, preferred orientation, species of the formed carbides, optical micro-structure, and thickness were
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55
examined by SEM and optical microscope observations, energy dispersive spectroscopy (EDS), X-ray diffraction, electron probe microanalysis (EPMA), and other methods. The weight change from the coating was also measured.
3. Results
3. I. Coating onto W1 steel 3.1.1. Initial stage of coating formation SEM observation of the specimen surface revealed that very small grains, smaller than 0.1 /am, deposited on the whole surface at the very early stage. Even in the shortest immersion time of 5 s at 1098 K, such depositions as those shown in Fig. l(a) were observed and vanadium was detected by EDS. Grains of 0.5-2 /am size appeared on the 20 s immersion specimen to generate the network-like depositions shown in Fig. l(b). The number and size of these grains increased with time and they covered the whole surface after between 60 s and 1.8 Ks as shown in Figs. l(c)-l(e). This was also clearly recognized in Figs. 2(a), 2(b), 2(d) and 2(f), showing the cross-sectional observation through an optical microscope. Finally, very fine grains of sub-micron size began to be discovered, as shown in Fig. l(e), on the surfaces which seemed to be almost completely covered with coarse grains and very smooth surfaces were made with longer immersion times as shown in Figs. l(f), 2(c), 2(e) and 2(g). X-ray diffraction lines of VC were clearly recognized on the specimen immersed for 60 s at 1098 K, the surface of which was covered by a number of grains growing on and within the networks, as shown in Fig. l(c). Probably, the grains formed by immersion for shorter than 60 s were also VC, although identification
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Fig. 1. Scanning electron micrographs o f the surface of Wl steel specimens immersed for a short time range: (a) 5 s, (b) 20 s, (c) 60 s, (d) 180 s, (e) 1.8 Ks, (f) 7.2 Ks, (g) 20 s, (h) 5s, in a bath with added V205 and B4C at: 1098 K ( a ) - ( t ) , 1183 K (g), and 1283 K (h).
56
T. Arai, S. Moriyama / Growth behaviour of VC coatings on steel substrates 30
/ O
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Fig. 2. Optical micrographs of cross-sections of WI steel specimens immersed for a short time range: (a) 300 s, (b) 1.8 Ks, (c) 7.2 Ks, (d) 180 s, (f) 60 s, (g) 5 s, in a bath with added V205 and B4C at: 1098 K (a)-(c), 1183 K (d)-(f) and 1283 K (g).
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Fig. 3. Thickness of carbide layers formed on W1 specimens vs. immersion time in a bath with added VzO5 and B4C.
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of VC was impossible because of diffraction lines of cementite which were superimposed. This behavior was common at other coating temperatures. However, the increased temperature accelerated the depositions. The network-like depositions generated during the immersion times as short as 20 s at 1183 K and 5 s at 1283 K are shown in Figs. l(g) and l(h). The time needed for covering the whole surface with coarsened grains decreased with increasing coating temperature, i.e. about 1.8 Ks at 1098 K, between 180 and 300 s at 1183 K, and shorter than 5 s at 1283 K. Sub-micron sized grains were generated earlier as the temperature increased, namely during 1.8-7.2 Ks at 1098 K, 30 s - l . 8 Ks at 1183 K, and 5 - 1 8 0 s at 1283 K. The same tendency was found for the time needed for obtaining clear diffraction lines of VC, which were recognized after 60 s at 1098 and 1183 K but after 5 s at 1283 K. From comparison of Figs. 2(b), 2(d) and 2(f), the size of the coarse grains before being covered with the sub-micron sized grains seems to become rather decreased with coating temperature. 3.1.2. Growth o f the coatings
The thickness of the coatings increased in proportion to the square root of the immersion time, as shown in Fig. 3, implying that the coating growth was achieved by the diffusion controlled reaction [13]. Thickness measurement with accuracy is relatively difficult by optical microscopic observation of the cross-sections,
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Fig. 4. Weight gain on Wl specimens by short time immersion vs. immersion time in a bath with added V205 and B4C.
especially in the initial stage of nucleation and growth. As shown in Fig. 4, however, weight gains at the initial stage also increased proportionally with the square root of immersion time, supporting the conclusion based on the thickness increase. SEM observation of the specimen surfaces immersed for a long time concluded that the coating temperature has a large effect on the size and shape of grains formed
T. Arai, S. Moriyama / Growth beha~iour of VC coatings on steel substrates
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Fig. 5. Scanning electron micrographs of surfaces of WI steel specimens immersed for a long time range: (a) 420 s, (b) 1.8 Ks, (c) 14.4 Ks, (d) 180 s, (e) 7.2 Ks, (f) and (g) 28.8 Ks, (h) 72 Ks, in a bath with added VzO5 and B4C at: 1183 K (a)-(c), 1283 K (d) and (e), and 1098 K (f)-(h).
during the longer immersion time. At 1183 K, the grains changed from very fine grains to equiaxed fine grains as shown in Figs. 5(a)-5(c). A large difference in size was not found between 14.4 Ks and 72 Ks. Small needle-like grains were found together with a large number of equiaxed grains at 1283 K as shown in Fig. 5(d), and both grains increased their size as shown in Fig. 5(e). After 14.4 Ks and 72 Ks, the needle-like grains were not discovered, although equiaxed grains still increased in size. The needle-like grains were not found at the other temperatures examined in this experiment. The size of the equiaxed grains formed at 1283 K was much larger than those formed at 1183 K in comparison with those at the same thickness level. This is supported by the observation of fracture morphology shown later. The lower temperature seems to be recommendable to obtain fine grains. However, the coating formed at 1098 and 1133 K by the long immersion at 28.8 and 72 Ks showed the undulation which is seen in Fig. 5(f) at the smaller magnification, and the "cauliflower" like and porous appearance shown in Figs. 5(g) and 5(h) at the higher magnification. The change in the peak ratio of I(111) and •(200) of VC with thickness of the coatings is illustrated in Fig. 6(a), where the values obtained from each specimen at various immersion time points were plotted against the thickness of the coatings formed on each specimen. This figure therefore shows distribution of the peak ratio in each coating. It can be considered, because there is no remarkable discontinuity of the curves, that each curve expresses the change of the peak ratios towards the surface from the substrate side in thick coatings. Very large peak values of the coating formed on Wl at 60 s rapidly decreased to very small values,
smaller than 0.1, as illustrated in Fig. 6(a), meaning a drastic change of preferred orientation from (111) to (200). The peak ratios obtained at other low temperatures, such as 1038, 1073, 1123, and 1133 K, agreed well with those at 1098 K. The curves for the peak ratio versus thickness at high coating temperatures, higher than 1173 K, were fairly different from those for the low temperatures. The peak ratio values are also larger than the ASTM bulk value for VC (14 wt.% C) and gradually decreased with increasing thickness to the constant value of about 0.9. Figure 7 illustrates the morphology of the fractured cross-sections of the specimens immersed at 1093, 1183 and 1283 K. Figure 7(a) shows that the thick coating formed at 1183 K has two distinct grain morphologies; a columnar region adjacent to the substrate, about 5 lam thick, and an upper fine-grained region. Coating at 1283 K was also composed of the two regions, although the columnar one was very thin, about 1 lam thick, as shown in Fig. 7(b). The columnar region appears to correspond with the coarse grain region formed in the initial stage from comparison of the thickness:-The equiaxed grains gradually increased their size with increased distance from the substrate both at 1183 and 1283 K in accordance with the results of SEM surface observation and changed their shape to a slightly elongated grain towards the surface at 1283 K. A very fine columnar structure was observed, as shown in Fig. 7(c), on the coating at 1098 K, most of which was formed in the initial stage. Comparison of the cross-sectional morphology with the change of the peak ratio versus the thickness of the coatings concluded that the strong preferred orientation was obtained from the columnar structure formed at
58
T. Arai, S. Moriyama / Growth behaviour of VC coatings on steel substrates
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Fig. 7. Fracture morphologies of WI steel specimens immersed in a bath with added V20 5 and B4C: (a) 1183 K, 72 Ks; (b) 1283 K, 1"4.4 Ks; (c) 1098 K, 7.2 Ks.
the initial stage and the random orientation from the equiaxed structure. Comparison of cross-sectional morphology between the coatings after immersion at 1183 K and 1283 K led to the conclusion that recrystallization of grains formed at the early stages did not occur during the further growth of the coatings. 3.2. Coating onto other substrates Figure 6(b) illustrates the change of the peak ratio with increasing thickness for D2 as well as the peak
ratio values obtained for M2 after immersion for 21.6 Ks at the different temperatures. Peak ratio values for 1045 are plotted in Fig. 6(a). The values for 1045 and M2 were plotted very close to the curves for W1 and D2, respectively. Thus, a large difference is recognized between carbon steels and highly alloyed tool steels. The curves for alloyed steels at the temperatures as low as 1038 to 1123 K rapidly increased from very small values to very large values with increasing thickness, on the contrary to carbon steels. The reversed tendency
T. Arai, S. Moriyama / Growth behaviour of VC coatings on steel substrates
was also observed in the thin coating range of the curves at high temperatures. In the thick range of the curves at high temperatures, higher than 1173 K, the coatings on alloyed steels showed I(111)/1(200) values close to the ASTM value, implying equiaxed grains. Actually, the fracture morphology of the coating on D2 and M2 at 1298 K had equiaxed grains of which size increased toward the surface. 1173 K was in the category of the high temperature in carbon steels and the low temperature in highly alloyed steels. It is very interesting that temperatures can be divided into the high temperatures and low temperatures both for carbon steels and alloyed steels between 1123 and 1173 or 1183 K. The plate-like grains were also recognized on the coatings on 1045, D2 and M2 specimens only at 1273 K as in the case of Wl. A large difference in grain size was not found between the substrate steels judging from SEM observations on the coating surfaces.
3.3. Effect of bath composition on growth of the coatings Figure 8 shows the effects of amounts of vanadium oxide and ferro-vanadium on thicknesses and 1(111)/ •(200) of the formed carbide coatings. The amount of vanadium-containing additives and the ratio of the reducing agent to vanadium oxide did not bring a large effect on the thickness except in the thin coatings o
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Fig. 8. Effect of bath composition on 1(111)/1(200) of VC and thickness of coatings: (a) bath with varied amounts of Fe-V, WI (O), 1045 (&) and D2 steel (x), 1223 K, 14.4 Ks; (b) bath with varied amounts ofV205 and 5 wt.% AI (O, A) or B4C ( 0 , A), Wl steel, 1173 and 1273 K, 7.2 Ks.
59
formed with a very small amount of ferro-vanadium. A change in I(111)/I(200) was also found only in the thin coatings. Surface and cross-sectional morphologies also did not change. It can be concluded from these results that the bath composition has no large effect on the thickness [2], and the micro-structure of the coatings except for case with very small amounts of additives.
4. Discussion The coatings were formed by the following steps at any temperature. Primary nucleation and growth. A number of sub-micron sized grains yielded on the substrate surface, and relatively coarse grains grew from these grains increasing their size and number with increasing immersion time until they covered the whole surface. This produced a thin region having columnar morphology and strong preferred orientation. (Hereafter, this region is called the first zone.) Generation of very fine grains on the surface of the first zone to produce a very smooth surface. (Hereafter called the second zone.) Growth of columnar or equiaxed grains on the second zone to make the thick layers. (Hereafter called the third zone.) During the first zone grains were still growing, and the grains in the second zone grew on the grains in the first zone which were locally crowded. Therefore, the formation of the second zone started before the formation of the first zone come to an end. The first zone may be attributable to selected growth of the grains having preferred orientation among many other randomly-orientated grains. The increased grain size in the third zone toward the surface may be caused by poorly-supplied carbon atoms for the reaction front owing to a longer diffusion path and the resulting lower nucleation. There may be no differences between the second zone and the third zone except grain size, and the second zone might even be considered to be part of the third zone. A paper on CVD carbide coating using a gas with no carbon-containing materials reported that chromium carbide grains generated on steel substrates at early stages of the deposition process in the same manner as in the first zone in the present paper [14]. However, the paper by Mlynarezak et al. [10] more closely relates to the present work. They reported the three zones very similar to those in the present work in their paper on VC coating on carbon steel and low allowed steel by the powder pack method, in which specimens needed a very long time, 1.08 Ks, to reach the coating temperature of 1273 K. They concluded that the first zone and the second zone were formed before the specimen reached
60
T. Arai, S. Morivama / Growth behaviour of VC coatings on steel substrates
the coating temperature, in close connection with the existence and its solution of cementite particles, and the third zone grew up at the coating temperature through the reaction of vanadium with carbon atoms supplied to the surface from the substrate's austenite phase. The behaviour of the formation of the first and the second zones in the present work may be not completely identical to that proposed for the pack process, because the first zone and the second zone in the present work grew after specimens reached the coating temperature. However, a small amount of undissolved cementite particles were observed in W1 by microscopic observations up to immersion periods almost similar to those in which the grains in the first zone covered the whole surface. The cementite particles may exert an influence, therefore, only at the very early stage of the first-zone formation, even though it was always possible. However, in the case of alloyed steels, the deposition of VC grains onto alloyed carbide particles [15] may have a large effect on the growth behavior. Alloyed carbides in alloyed steels are known to be different from cementite in their affinity to carbon and the dissolving behavior into the steel matrix, and other points. This may result in the different preferred orientation observed in the present work and other behaviors of the initial nucleation and growth, and successive growth to thick layers. This question is the subject of further study. Other possible controlling factors for the morphology and preferred orientation are bath composition and coating temperature. Vanadium-containing materials were added in baths only on the occasion of bath preparation unlike CVD. Therefore, no decrease in reactive atoms to combine with carbon atoms should be ensured during the whole coating cycles. Possible causes of the decrease are consumption by carbide formation and the change of vanadium from reactive species to the interactive species, namely bath deterioration. As long as coating thickness should be a subject, these need not be considered because the amount and type of vanadiumcontaining additives adopted here were determined through extensive studies to maintain the constant thickness. Although few studies have been made concerning the morphology and preferred orientation in relation to this question, the results for ferro-vanadium shown in Fig. 8 will suggest that a serious effect cannot be expected on these items, as long as baths ensure the constant thickness. As for coating temperature, a possible effect of the temperature on the amount of the reactive vanadium in the bath should be considered. Only a part of the vanadium atoms added in the bath can be expected to be available as the reactant for carbide formation, although the reactant species has not yet been confirmed [16]. The solubility of vanadium in borax in the metal powder system, the equilibrium constant in the vana-
dium oxide and reducing agent system, the reaction of the vanadium-containing additive with oxygen, and other factors are supposed to change with bath temperature, having an influence on the amount of the reactant. However, this may not be so from the results shown in Fig. 8 and the change in growth behavior with temperature obtained in this work is mainly attributable to the temperature-dependency of diffusion, etc., as in the case of CVD. Coatings grew with (111) preferred orientation on carbon steels and with (200) on alloyed steels at low temperatures, whereas at high temperatures coatings were randomly orientated on both steels except near the substrate. The effect of the micro-structure, including the preferred orientation, on various properties of the coatings is also expected for these coatings as has already been observed in CVD and physical vapour deposition. This should be a subject for further research. Very large changes in the grain morphology and the preferred orientation were found throughout the thickness of the coatings formed by this coating process, unlike the ordinary CVD carbide coating in which the differences are usually found between the thin layer formed adjacent to the substrate with carbon supplied from substrate and the layer formed thereafter with carbon in the gas [17-19], and no large difference in grain size is usually observed [20]. This peculiarity is attributable to the fact that, in this coating process, carbon is exclusively supplied from the substrate and the supply of carbon to the reaction front, the surface of the carbide layer, decreases with an increase of the thickness because of the increase of the diffusion path. The changes of the grain morphology and the preferred orientation were also highly influenced by the substrate steel and the coating temperature. This fact should be kept in consideration to select the coating temperature and the coating thickness in practical applications. There is almost no detailed information about the micro-structure and preferred orientation of thick carbide coatings by CVD with no supply of carbon in the reaction gas, although some papers [21-23] have dealed with thick coatings of titanium carbide, chromium carbide, and VC on steel substrates as well as paper [14]. The results obtained in the present paper are expected to include some knowledge common to thick coating by CVD with no carbon supply.
5. Conclusion
1. There are very large changes in the grain morphology and the preferred orientation throughout the thickness of the coatings depending on the substrate steels and the coating temperature. 2. VC coatings were formed through three stages: primary nucleation and growth, generation of very fine
7". Arai, S. Moriyama / Growth behaviour o f VC coatings on steel substrates
grains thereon, and growth to thick coatings by successive deposition of columnar grains with strong preferred orientations or equiaxed grains with random orientations. 3. The columnar grains were formed at low temperatures and the equiaxed grains at high temperatures. 4. Preferred orientations of the coatings for highly alloyed steels, D2 ~and M2, were considerably different from those for carbon steels, W1 and 1045.
Acknowledgment The authors wish to thank many members of Toyota Central Research and Development Labs., Inc. for their conducting the present experimental program.
References 1 T. Arai, in T. Sudarshan and D. G. Bhat (eds.), Surface Modification Technologies, TMS Proc. 3rd Int. Conf., Neuchatel, Switzerland, 1989, p.587. 2 T. Arai, H. Fujita, Y. Sugimoto and Y. Ohta, J. Mater. Eng., 9 (1987) 183. 3 K. Saruki, S. Hotta, H. Fujita and T. Arai, Thin Solid Films, 181 (1989) 383. 4 D. H. Jang and J. S. Chun, Thin Solid Fihns, 149 (1987) 95.
61
5 I. J. Konyashin, Y. M. Korelev, A. I. Anikeev, A. A. Korchangin and A. N. Zarakhani, Proc. 12th Int. Plansee Seminar 1989, Vol.3, 1989, Reutte, Tirol, Austria, p.119. 6 C. W. Lee, S. W. Nam and J. S. Chun, Thin Solid Films, 86(1981) 63. 7 J. N. Lindstrom and R. Johannesson, J. Electrochem. Soc., 123 (1976) 555. 8 0 . V. Roman, L. M. Kirilyuk, G. N. Dubrorvskaya, V. N. Anikin and A. I. Anikeyev, Powder Met. Int., 13 (1981) 192. 9 J. Skogsmo, A. Henjered and H. Norden, R. HM., 6 (1987) 84. I0 A. Mlynarezak and K. Jastrzebowski, Neue Hutte, 25 (1980) 259. 11 Y. Komen, B. Z. Weiss and S. Niedzwiez, J. Iron Steel lnst., (1968) 487 12 Z. Glowachi and K. Jastrzebowski, Neue Hutte, 29 (1984) 220. 13 T, Arai, H. Fujita, M. Mizutani and N. Komatsu, J. Jpn. Inst. Metals, 40 (1976) 925. 14 A. J. Perry and E. Horbath, Proc. 7th Int. Conf. Chemical Vapor Deposition, Los Angeles, CA, USA, 1976. 15 T. Arai, Thin Solid Films, in press. 16 H. C. Child, S. A. Plumb and J. J. Mcdermott, Proc. Int. Conf. Heat Treatment 1984, Heat Treatment Committ. Metals Soc., London, 1984, p.51. 17 J. Liao, J. Li and K. H. Kuo, Met. Trans. A., 20A (1989) 279. 18 S. Vuorinen and A. Horsewell, J. Mater. Sci., 17 (1982) 589. 19 A. Leonhardt, D. Schl/ifer, M. Seidler, D. Selbmann and M. Sch6nherr, J. Less-Common Metals, 87 (1982) 63. 20 W. G. Sloof, R. Delhez, Th. H. de Keijser, D. Schalkoord, P. P. J. Ramekers and G. F. Bastin, J. Mater. Sci., 23 (1988) 1660. 21 S. G. Yoon, H. G. Kim and J. S. Chun, J. Mater. Sci., 22 (1987) 2629. 22 A. J. Perry, Wear, 67 (1981) 38 I. 23 E. Horvath and A. J. Perry, Thin Solid Films, 65 (1980) 309.
54
Growth behavior of vanadium carbide coatings on steel substrates by a salt bath immersion coating process Tohru Arai and Sigeo Moriyama Toyota Central Research and Development Labs., Inc. 41-1, Yokomichi, Nagakute, Nagakute-cho, Aichi-Gun, Aichi-Ken (Japan) (Received July 20, 1993; accepted February 16, 1994)
Abstract Nucleation and growth behavior, micro-structure, and preferred orientation of vanadium carbide coatings formed using a borax bath were studied, focussing on the effects of coating temperature and substrate steel. The coatings were formed through three stages; nucleation and growth of relatively coarse grains, sub-micron sized grain formation on these, and growth into thick coatings by successive deposition of coarse grains. Preferred orientation was highly influenced by temperature and substrate steel.
1. Introduction In the thermo-reactive deposition and diffusion (TRD) carbide coating process [1] using a borax bath containing no carbon, carbide coatings grow onto carbon-containing substrates through the combination of carbide-forming-element atoms in the bath with carbon atoms exclusively supplied from the substrates. A number of papers have been published about this coating process to clarify various points, such as the reaction in the borax salt, the mechanism of carbide layer formation, factors controlling the growth rate of carbide layers, properties of carbide layers and carbide-coated materials [1-3]. For better understanding of the coating formation and properties of the coatings, however, the micro-structure of the coatings (the shapes and sizes of grains, preferred orientations, etc.) as a result of heterogeneous nucleation and growth processes must be examined. The purpose of this paper is, therefore, to show the results of experiments on nucleation and growth of vanadium carbide (VC) coatings onto steel substrates. Initial nucleation and growth at the coating-substrate interface has not been well understood in this coating process. There have been a number of research papers on the nucleation and growth during chemical vapour deposition (CVD) [4-9], and very few papers have dealt with this in the diffusion carbide coating by the powder pack method and the gas method [10-12]. The salt bath immersion process in ambient atmosphere has two advantages over CVD and the powder method in observing the very early stage of coating deposition. It is much easier to coat specimens within a very short immersion time if the specimens are sufficiently small to rapidly
0040-6090/94/$7.00
SSDI 0040-6090(94) 06105-T
reach the bath temperature. The shortest time used in this research was 5 s, which was employed in very few of the papers on CVD [5]. The other advantage is the capability of rapid cooling from the coating temperature to quench the growth pattern. In contrast, if the salt agent remained on the specimen surfaces it may disturb surface observations. This can be solved by well-managed cleaning operation and careful observation.
2. Experimental details
2.1. Carbide coating procedure VC coating was carried out by the salt bath immersion process [1]. Polished small specimens of AISI 1045, W1 (0.95 wt.% C), D2, and M2 steels in the annealed state were immersed for the specified time in borax baths with added vanadium penta-oxide flakes and boron carbide powders or aluminum pieces as reducing agents for vanadium oxide, or in baths with ferro-vanadium powders, and then air cooled or quenched in water. Differing the cooling methods did not produce any serious effect on the results. A bath containing 20 wt.% vanadium penta-oxide and 5 wt.% boron carbide was mainly used. A bath with varied amounts of vanadium penta-oxide and boron carbide or aluminum, and a bath with varied amounts of ferro-vanadium were employed only for examining the effect of bath composition. The baths were kept at 1038-1298 K in ambient atmosphere. A very wide range of immersion times, 5-86.4 Ks, was adopted for W1 for the detailed examination of the initial stage of coating growth. An immersion time
~L 1994 - - Elsevier Science S.A. All rights reserved
T. Arai, S. Moriyama / Growth behaviour o f VC coatings on steel substrates
range of 300-86.4 Ks was employed for D2 to examine preferred orientation. 1045 and M2 were immersed only for 21.6 Ks for comparison with Wl and D2. Thin specimens, 1 mm thick and 8 mm in diameter, were used for the immersion times shorter than 3.6 Ks to ensure a rapid rise of specimen temperature. Specimens 15 mm long and 8 mm in diameter, were used for a longer immersion time so that the specimens contain sufficient carbon atoms for thicker coating formation. Temperature measurement using specimens with a welded thermocouple, having diameters of 0.64 mm, concluded that 1 mm thick specimens reached a bath temperature of 1188 K after about 15 s and 15 mm long specimens after about 180 s. It was roughly estimated that 1 mm thick specimens reached above 1073 K and 15 mm long specimens 573 K after 5 s immersion in a 1188 K bath. Scanning electron microscopy (SEM) observations of fractured cross-sections of W1 steel coated at 1183 and 1283 K showed that the morphology of the coating on 1 mm thick specimens by immersion for 1.8 Ks was in good accordance with those on 15 mm long specimens by immersions for 7.2, 14.4, and 72 Ks. Also, no remarkable discontinuity was found on the curves showing the change of preferred orientation with increased thickness. It is concluded from this evidence that the differences in specimen size and the resulting heating-up time did not affect the purpose of this research.
2.2. Observations The bath salt attached to the specimens was washed off by immersion in hot water, followed by ultrasonic cleaning in ethanol and acetone. SEM surface observation was carried out mainly on W1 specimens. Morphology, preferred orientation, species of the formed carbides, optical micro-structure, and thickness were
f a ) ~
1 t~ m i
55
examined by SEM and optical microscope observations, energy dispersive spectroscopy (EDS), X-ray diffraction, electron probe microanalysis (EPMA), and other methods. The weight change from the coating was also measured.
3. Results
3. I. Coating onto W1 steel 3.1.1. Initial stage of coating formation SEM observation of the specimen surface revealed that very small grains, smaller than 0.1 /am, deposited on the whole surface at the very early stage. Even in the shortest immersion time of 5 s at 1098 K, such depositions as those shown in Fig. l(a) were observed and vanadium was detected by EDS. Grains of 0.5-2 /am size appeared on the 20 s immersion specimen to generate the network-like depositions shown in Fig. l(b). The number and size of these grains increased with time and they covered the whole surface after between 60 s and 1.8 Ks as shown in Figs. l(c)-l(e). This was also clearly recognized in Figs. 2(a), 2(b), 2(d) and 2(f), showing the cross-sectional observation through an optical microscope. Finally, very fine grains of sub-micron size began to be discovered, as shown in Fig. l(e), on the surfaces which seemed to be almost completely covered with coarse grains and very smooth surfaces were made with longer immersion times as shown in Figs. l(f), 2(c), 2(e) and 2(g). X-ray diffraction lines of VC were clearly recognized on the specimen immersed for 60 s at 1098 K, the surface of which was covered by a number of grains growing on and within the networks, as shown in Fig. l(c). Probably, the grains formed by immersion for shorter than 60 s were also VC, although identification
u ra
Fig. 1. Scanning electron micrographs o f the surface of Wl steel specimens immersed for a short time range: (a) 5 s, (b) 20 s, (c) 60 s, (d) 180 s, (e) 1.8 Ks, (f) 7.2 Ks, (g) 20 s, (h) 5s, in a bath with added V205 and B4C at: 1098 K ( a ) - ( t ) , 1183 K (g), and 1283 K (h).
56
T. Arai, S. Moriyama / Growth behaviour of VC coatings on steel substrates 30
/ O
"
(f)
~0
/
/
//
/
~St~m
vc 0
Fig. 2. Optical micrographs of cross-sections of WI steel specimens immersed for a short time range: (a) 300 s, (b) 1.8 Ks, (c) 7.2 Ks, (d) 180 s, (f) 60 s, (g) 5 s, in a bath with added V205 and B4C at: 1098 K (a)-(c), 1183 K (d)-(f) and 1283 K (g).
i
2
;ubstrate
4 Immersion time
h I/2
Fig. 3. Thickness of carbide layers formed on W1 specimens vs. immersion time in a bath with added VzO5 and B4C.
1.5
of VC was impossible because of diffraction lines of cementite which were superimposed. This behavior was common at other coating temperatures. However, the increased temperature accelerated the depositions. The network-like depositions generated during the immersion times as short as 20 s at 1183 K and 5 s at 1283 K are shown in Figs. l(g) and l(h). The time needed for covering the whole surface with coarsened grains decreased with increasing coating temperature, i.e. about 1.8 Ks at 1098 K, between 180 and 300 s at 1183 K, and shorter than 5 s at 1283 K. Sub-micron sized grains were generated earlier as the temperature increased, namely during 1.8-7.2 Ks at 1098 K, 30 s - l . 8 Ks at 1183 K, and 5 - 1 8 0 s at 1283 K. The same tendency was found for the time needed for obtaining clear diffraction lines of VC, which were recognized after 60 s at 1098 and 1183 K but after 5 s at 1283 K. From comparison of Figs. 2(b), 2(d) and 2(f), the size of the coarse grains before being covered with the sub-micron sized grains seems to become rather decreased with coating temperature. 3.1.2. Growth o f the coatings
The thickness of the coatings increased in proportion to the square root of the immersion time, as shown in Fig. 3, implying that the coating growth was achieved by the diffusion controlled reaction [13]. Thickness measurement with accuracy is relatively difficult by optical microscopic observation of the cross-sections,
"2 1.0
/
O
-
.=_
-
e,0
E o ~
0.5
.2 0
i
0
i
,
i
2
i
0
4 Immersion
time
mill I/2
Fig. 4. Weight gain on Wl specimens by short time immersion vs. immersion time in a bath with added V205 and B4C.
especially in the initial stage of nucleation and growth. As shown in Fig. 4, however, weight gains at the initial stage also increased proportionally with the square root of immersion time, supporting the conclusion based on the thickness increase. SEM observation of the specimen surfaces immersed for a long time concluded that the coating temperature has a large effect on the size and shape of grains formed
T. Arai, S. Moriyama / Growth beha~iour of VC coatings on steel substrates
" ",
g
.
(a)
--1
,,'
~
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-
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um
~
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,
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.111 C d _ ) _ ~ L ~ - - i
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~
2
57
15ts m ( g ) ~ w
20k'j
It'~ ~
L
1 /~ m__ ' , h ) - - ~ - - -
1 ,um
Fig. 5. Scanning electron micrographs of surfaces of WI steel specimens immersed for a long time range: (a) 420 s, (b) 1.8 Ks, (c) 14.4 Ks, (d) 180 s, (e) 7.2 Ks, (f) and (g) 28.8 Ks, (h) 72 Ks, in a bath with added VzO5 and B4C at: 1183 K (a)-(c), 1283 K (d) and (e), and 1098 K (f)-(h).
during the longer immersion time. At 1183 K, the grains changed from very fine grains to equiaxed fine grains as shown in Figs. 5(a)-5(c). A large difference in size was not found between 14.4 Ks and 72 Ks. Small needle-like grains were found together with a large number of equiaxed grains at 1283 K as shown in Fig. 5(d), and both grains increased their size as shown in Fig. 5(e). After 14.4 Ks and 72 Ks, the needle-like grains were not discovered, although equiaxed grains still increased in size. The needle-like grains were not found at the other temperatures examined in this experiment. The size of the equiaxed grains formed at 1283 K was much larger than those formed at 1183 K in comparison with those at the same thickness level. This is supported by the observation of fracture morphology shown later. The lower temperature seems to be recommendable to obtain fine grains. However, the coating formed at 1098 and 1133 K by the long immersion at 28.8 and 72 Ks showed the undulation which is seen in Fig. 5(f) at the smaller magnification, and the "cauliflower" like and porous appearance shown in Figs. 5(g) and 5(h) at the higher magnification. The change in the peak ratio of I(111) and •(200) of VC with thickness of the coatings is illustrated in Fig. 6(a), where the values obtained from each specimen at various immersion time points were plotted against the thickness of the coatings formed on each specimen. This figure therefore shows distribution of the peak ratio in each coating. It can be considered, because there is no remarkable discontinuity of the curves, that each curve expresses the change of the peak ratios towards the surface from the substrate side in thick coatings. Very large peak values of the coating formed on Wl at 60 s rapidly decreased to very small values,
smaller than 0.1, as illustrated in Fig. 6(a), meaning a drastic change of preferred orientation from (111) to (200). The peak ratios obtained at other low temperatures, such as 1038, 1073, 1123, and 1133 K, agreed well with those at 1098 K. The curves for the peak ratio versus thickness at high coating temperatures, higher than 1173 K, were fairly different from those for the low temperatures. The peak ratio values are also larger than the ASTM bulk value for VC (14 wt.% C) and gradually decreased with increasing thickness to the constant value of about 0.9. Figure 7 illustrates the morphology of the fractured cross-sections of the specimens immersed at 1093, 1183 and 1283 K. Figure 7(a) shows that the thick coating formed at 1183 K has two distinct grain morphologies; a columnar region adjacent to the substrate, about 5 lam thick, and an upper fine-grained region. Coating at 1283 K was also composed of the two regions, although the columnar one was very thin, about 1 lam thick, as shown in Fig. 7(b). The columnar region appears to correspond with the coarse grain region formed in the initial stage from comparison of the thickness:-The equiaxed grains gradually increased their size with increased distance from the substrate both at 1183 and 1283 K in accordance with the results of SEM surface observation and changed their shape to a slightly elongated grain towards the surface at 1283 K. A very fine columnar structure was observed, as shown in Fig. 7(c), on the coating at 1098 K, most of which was formed in the initial stage. Comparison of the cross-sectional morphology with the change of the peak ratio versus the thickness of the coatings concluded that the strong preferred orientation was obtained from the columnar structure formed at
58
T. Arai, S. Moriyama / Growth behaviour of VC coatings on steel substrates
(a)
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-~
1.0
1083 K W1 1073 0 1098 1123 a 1133 • 1173 D 1183 m 1223 o 1273 v 1283 •
(b)
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o
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~ [] ° v
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ASTM
--
,v
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Fig. 6. Relation between 1(111)/1(200) of VC and thickness obtained from each steel specimen immersed for various periods in a bath with added V205 and B4C.
Substrate- I
Substrato I
VC
VC
"1
4 SobstratoP VC
Fig. 7. Fracture morphologies of WI steel specimens immersed in a bath with added V20 5 and B4C: (a) 1183 K, 72 Ks; (b) 1283 K, 1"4.4 Ks; (c) 1098 K, 7.2 Ks.
the initial stage and the random orientation from the equiaxed structure. Comparison of cross-sectional morphology between the coatings after immersion at 1183 K and 1283 K led to the conclusion that recrystallization of grains formed at the early stages did not occur during the further growth of the coatings. 3.2. Coating onto other substrates Figure 6(b) illustrates the change of the peak ratio with increasing thickness for D2 as well as the peak
ratio values obtained for M2 after immersion for 21.6 Ks at the different temperatures. Peak ratio values for 1045 are plotted in Fig. 6(a). The values for 1045 and M2 were plotted very close to the curves for W1 and D2, respectively. Thus, a large difference is recognized between carbon steels and highly alloyed tool steels. The curves for alloyed steels at the temperatures as low as 1038 to 1123 K rapidly increased from very small values to very large values with increasing thickness, on the contrary to carbon steels. The reversed tendency
T. Arai, S. Moriyama / Growth behaviour of VC coatings on steel substrates
was also observed in the thin coating range of the curves at high temperatures. In the thick range of the curves at high temperatures, higher than 1173 K, the coatings on alloyed steels showed I(111)/1(200) values close to the ASTM value, implying equiaxed grains. Actually, the fracture morphology of the coating on D2 and M2 at 1298 K had equiaxed grains of which size increased toward the surface. 1173 K was in the category of the high temperature in carbon steels and the low temperature in highly alloyed steels. It is very interesting that temperatures can be divided into the high temperatures and low temperatures both for carbon steels and alloyed steels between 1123 and 1173 or 1183 K. The plate-like grains were also recognized on the coatings on 1045, D2 and M2 specimens only at 1273 K as in the case of Wl. A large difference in grain size was not found between the substrate steels judging from SEM observations on the coating surfaces.
3.3. Effect of bath composition on growth of the coatings Figure 8 shows the effects of amounts of vanadium oxide and ferro-vanadium on thicknesses and 1(111)/ •(200) of the formed carbide coatings. The amount of vanadium-containing additives and the ratio of the reducing agent to vanadium oxide did not bring a large effect on the thickness except in the thin coatings o
A x
W1 1045 D2
(a)
(b) O& IIA
5mass% AI,WI 5mass%BaC, W1
1.2
--~" ~-~-oa---:~ 0.8
..... • -ASTM O - ~ - 0 - -- ~ 0 1273K
(
&, 0.4
x• •
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"a.
0
8
ooo
,
o
, ,
io Fe.V
1273K
~ ,
1173K
,
,/o mass%
o""~ ° ' ~
,
~o V205
20 mass%
Fig. 8. Effect of bath composition on 1(111)/1(200) of VC and thickness of coatings: (a) bath with varied amounts of Fe-V, WI (O), 1045 (&) and D2 steel (x), 1223 K, 14.4 Ks; (b) bath with varied amounts ofV205 and 5 wt.% AI (O, A) or B4C ( 0 , A), Wl steel, 1173 and 1273 K, 7.2 Ks.
59
formed with a very small amount of ferro-vanadium. A change in I(111)/I(200) was also found only in the thin coatings. Surface and cross-sectional morphologies also did not change. It can be concluded from these results that the bath composition has no large effect on the thickness [2], and the micro-structure of the coatings except for case with very small amounts of additives.
4. Discussion The coatings were formed by the following steps at any temperature. Primary nucleation and growth. A number of sub-micron sized grains yielded on the substrate surface, and relatively coarse grains grew from these grains increasing their size and number with increasing immersion time until they covered the whole surface. This produced a thin region having columnar morphology and strong preferred orientation. (Hereafter, this region is called the first zone.) Generation of very fine grains on the surface of the first zone to produce a very smooth surface. (Hereafter called the second zone.) Growth of columnar or equiaxed grains on the second zone to make the thick layers. (Hereafter called the third zone.) During the first zone grains were still growing, and the grains in the second zone grew on the grains in the first zone which were locally crowded. Therefore, the formation of the second zone started before the formation of the first zone come to an end. The first zone may be attributable to selected growth of the grains having preferred orientation among many other randomly-orientated grains. The increased grain size in the third zone toward the surface may be caused by poorly-supplied carbon atoms for the reaction front owing to a longer diffusion path and the resulting lower nucleation. There may be no differences between the second zone and the third zone except grain size, and the second zone might even be considered to be part of the third zone. A paper on CVD carbide coating using a gas with no carbon-containing materials reported that chromium carbide grains generated on steel substrates at early stages of the deposition process in the same manner as in the first zone in the present paper [14]. However, the paper by Mlynarezak et al. [10] more closely relates to the present work. They reported the three zones very similar to those in the present work in their paper on VC coating on carbon steel and low allowed steel by the powder pack method, in which specimens needed a very long time, 1.08 Ks, to reach the coating temperature of 1273 K. They concluded that the first zone and the second zone were formed before the specimen reached
60
T. Arai, S. Morivama / Growth behaviour of VC coatings on steel substrates
the coating temperature, in close connection with the existence and its solution of cementite particles, and the third zone grew up at the coating temperature through the reaction of vanadium with carbon atoms supplied to the surface from the substrate's austenite phase. The behaviour of the formation of the first and the second zones in the present work may be not completely identical to that proposed for the pack process, because the first zone and the second zone in the present work grew after specimens reached the coating temperature. However, a small amount of undissolved cementite particles were observed in W1 by microscopic observations up to immersion periods almost similar to those in which the grains in the first zone covered the whole surface. The cementite particles may exert an influence, therefore, only at the very early stage of the first-zone formation, even though it was always possible. However, in the case of alloyed steels, the deposition of VC grains onto alloyed carbide particles [15] may have a large effect on the growth behavior. Alloyed carbides in alloyed steels are known to be different from cementite in their affinity to carbon and the dissolving behavior into the steel matrix, and other points. This may result in the different preferred orientation observed in the present work and other behaviors of the initial nucleation and growth, and successive growth to thick layers. This question is the subject of further study. Other possible controlling factors for the morphology and preferred orientation are bath composition and coating temperature. Vanadium-containing materials were added in baths only on the occasion of bath preparation unlike CVD. Therefore, no decrease in reactive atoms to combine with carbon atoms should be ensured during the whole coating cycles. Possible causes of the decrease are consumption by carbide formation and the change of vanadium from reactive species to the interactive species, namely bath deterioration. As long as coating thickness should be a subject, these need not be considered because the amount and type of vanadiumcontaining additives adopted here were determined through extensive studies to maintain the constant thickness. Although few studies have been made concerning the morphology and preferred orientation in relation to this question, the results for ferro-vanadium shown in Fig. 8 will suggest that a serious effect cannot be expected on these items, as long as baths ensure the constant thickness. As for coating temperature, a possible effect of the temperature on the amount of the reactive vanadium in the bath should be considered. Only a part of the vanadium atoms added in the bath can be expected to be available as the reactant for carbide formation, although the reactant species has not yet been confirmed [16]. The solubility of vanadium in borax in the metal powder system, the equilibrium constant in the vana-
dium oxide and reducing agent system, the reaction of the vanadium-containing additive with oxygen, and other factors are supposed to change with bath temperature, having an influence on the amount of the reactant. However, this may not be so from the results shown in Fig. 8 and the change in growth behavior with temperature obtained in this work is mainly attributable to the temperature-dependency of diffusion, etc., as in the case of CVD. Coatings grew with (111) preferred orientation on carbon steels and with (200) on alloyed steels at low temperatures, whereas at high temperatures coatings were randomly orientated on both steels except near the substrate. The effect of the micro-structure, including the preferred orientation, on various properties of the coatings is also expected for these coatings as has already been observed in CVD and physical vapour deposition. This should be a subject for further research. Very large changes in the grain morphology and the preferred orientation were found throughout the thickness of the coatings formed by this coating process, unlike the ordinary CVD carbide coating in which the differences are usually found between the thin layer formed adjacent to the substrate with carbon supplied from substrate and the layer formed thereafter with carbon in the gas [17-19], and no large difference in grain size is usually observed [20]. This peculiarity is attributable to the fact that, in this coating process, carbon is exclusively supplied from the substrate and the supply of carbon to the reaction front, the surface of the carbide layer, decreases with an increase of the thickness because of the increase of the diffusion path. The changes of the grain morphology and the preferred orientation were also highly influenced by the substrate steel and the coating temperature. This fact should be kept in consideration to select the coating temperature and the coating thickness in practical applications. There is almost no detailed information about the micro-structure and preferred orientation of thick carbide coatings by CVD with no supply of carbon in the reaction gas, although some papers [21-23] have dealed with thick coatings of titanium carbide, chromium carbide, and VC on steel substrates as well as paper [14]. The results obtained in the present paper are expected to include some knowledge common to thick coating by CVD with no carbon supply.
5. Conclusion
1. There are very large changes in the grain morphology and the preferred orientation throughout the thickness of the coatings depending on the substrate steels and the coating temperature. 2. VC coatings were formed through three stages: primary nucleation and growth, generation of very fine
7". Arai, S. Moriyama / Growth behaviour o f VC coatings on steel substrates
grains thereon, and growth to thick coatings by successive deposition of columnar grains with strong preferred orientations or equiaxed grains with random orientations. 3. The columnar grains were formed at low temperatures and the equiaxed grains at high temperatures. 4. Preferred orientations of the coatings for highly alloyed steels, D2 ~and M2, were considerably different from those for carbon steels, W1 and 1045.
Acknowledgment The authors wish to thank many members of Toyota Central Research and Development Labs., Inc. for their conducting the present experimental program.
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61
5 I. J. Konyashin, Y. M. Korelev, A. I. Anikeev, A. A. Korchangin and A. N. Zarakhani, Proc. 12th Int. Plansee Seminar 1989, Vol.3, 1989, Reutte, Tirol, Austria, p.119. 6 C. W. Lee, S. W. Nam and J. S. Chun, Thin Solid Films, 86(1981) 63. 7 J. N. Lindstrom and R. Johannesson, J. Electrochem. Soc., 123 (1976) 555. 8 0 . V. Roman, L. M. Kirilyuk, G. N. Dubrorvskaya, V. N. Anikin and A. I. Anikeyev, Powder Met. Int., 13 (1981) 192. 9 J. Skogsmo, A. Henjered and H. Norden, R. HM., 6 (1987) 84. I0 A. Mlynarezak and K. Jastrzebowski, Neue Hutte, 25 (1980) 259. 11 Y. Komen, B. Z. Weiss and S. Niedzwiez, J. Iron Steel lnst., (1968) 487 12 Z. Glowachi and K. Jastrzebowski, Neue Hutte, 29 (1984) 220. 13 T, Arai, H. Fujita, M. Mizutani and N. Komatsu, J. Jpn. Inst. Metals, 40 (1976) 925. 14 A. J. Perry and E. Horbath, Proc. 7th Int. Conf. Chemical Vapor Deposition, Los Angeles, CA, USA, 1976. 15 T. Arai, Thin Solid Films, in press. 16 H. C. Child, S. A. Plumb and J. J. Mcdermott, Proc. Int. Conf. Heat Treatment 1984, Heat Treatment Committ. Metals Soc., London, 1984, p.51. 17 J. Liao, J. Li and K. H. Kuo, Met. Trans. A., 20A (1989) 279. 18 S. Vuorinen and A. Horsewell, J. Mater. Sci., 17 (1982) 589. 19 A. Leonhardt, D. Schl/ifer, M. Seidler, D. Selbmann and M. Sch6nherr, J. Less-Common Metals, 87 (1982) 63. 20 W. G. Sloof, R. Delhez, Th. H. de Keijser, D. Schalkoord, P. P. J. Ramekers and G. F. Bastin, J. Mater. Sci., 23 (1988) 1660. 21 S. G. Yoon, H. G. Kim and J. S. Chun, J. Mater. Sci., 22 (1987) 2629. 22 A. J. Perry, Wear, 67 (1981) 38 I. 23 E. Horvath and A. J. Perry, Thin Solid Films, 65 (1980) 309.