Morphological modification of TiC by laser irradiation

Surface and Coatings Technology, 45 (1991) 393-397

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Morphological modification of TiC by laser irradiation A. B~ichli a n d A. Blatter Institute of Applied Physics, University of Berne, CH-3012 Berne (Switzerland)

Abstract The technique of laser quenching has been applied to TiC coatings on cemented carbide WC/Co material. In contrast to the original columnar morphology of TiC prepared by chemical vapour deposition, the laser-processed surface is isotropic and flat. With the proper parameters a smooth interface is formed and the TiC is alloyed to the substrate. The chemical composition as well as the beneficial tribological properties can be preserved.

1. I n t r o d u c t i o n H i g h - p o w e r lasers are i n c r e a s i n g l y u s e d in m e t a l l u r g y to m e l t a n d r a p i d l y resolidify the n e a r - s u r f a c e r e g i o n s of m a t e r i a l s w i t h o u t t h e r m a l l y or m e c h a n i c a l l y l o a d i n g the u n d e r l y i n g b u l k [1]. T h e m i c r o s t r u c t u r a l modifications a c h i e v e d r e s u l t in i m p r o v e d p e r f o r m a n c e in m a n y a p p l i c a t i o n s . TiC c o a t i n g s m a d e by c h e m i c a l v a p o u r d e p o s i t i o n are well k n o w n to r e d u c e w e a r a n d friction [2]. H o w e v e r , the deposit exhibits a c o l u m n a r m o r p h o l o g y w h i c h is d e t r i m e n t a l to p r o p e r t i e s s u c h as s h e a r s t r e n g t h , a d h e s i o n a n d c h e m i c a l resistance. We h a v e applied the l a s e r t e c h n i q u e to s u c h h i g h m e l t i n g p o i n t c o a t i n g s a n d r e p o r t h e r e on t h e m i c r o s t r u c t u r a l m o d i f i c a t i o n s achieved.

2. E x p e r i m e n t a l details TiC c o a t i n g s of t h i c k n e s s 3.4 p m w e r e g r o w n on W C / C o c e m e n t e d carbide by c h e m i c a l v a p o u r d e p o s i t i o n (CVD). T h e c o a t i n g s w e r e t h e n i r r a d i a t e d w i t h 50 ns pulses f r o m a Q-switched Nd: YAG l a s e r w i t h fluences b e t w e e n 1 a n d 15 J cm -2. F o r c o m p a r i s o n the focused b e a m of a CO2 l a s e r was s c a n n e d o v e r the m a t e r i a l (5 × 10 GW cm e). T h e r e s u l t i n g m i c r o s t r u c t u r e s w e r e e x a m i n e d by X-ray d i f f r a c t o m e t r y u s i n g Cu K~I r a d i a t i o n a n d s c a n n i n g e l e c t r o n m i c r o s c o p y (SEM). E n e r g y d i s p e r s i v e X-ray a n a l y s i s (EDX) a n d A u g e r e l e c t r o n s p e c t r o s c o p y (AES) w e r e e m p l o y e d for c h e m i c a l analysis. Sliding friction a n d w e a r m e a s u r e m e n t s w e r e p e r f o r m e d in air a t 99% humidity. P o l i s h e d TiC a n d steel balls of r a d i u s 3 m m w e r e p r e s s e d o n t o the s a m p l e s w i t h a static l o a d of 5 N. Elsevier Sequoia/Printed in The Netherlands

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Fig. 1. Scanning electron micrographs of a TiC coating (a) before and (b) after modificationwith an Nd: YAG laser pulse at 2 J cm-2.

3. R e s u l t s The microstructure of the TiC layer initially formed by CVD was essentially of columnar morphology (Fig. l(a)). The anisotropic grains with dimensions between 0.1 and 2 pm gave rise to a pronounced microroughness and the surface appeared black. A metallic .shiny smooth and homogeneous surface was obtained after remelting with the Nd: YAG laser operated between 2 and 8 J cm -2 (Fig. l(b)). The discernible network of microcracks was extremely fine, about two orders of magnitude finer than in as-deposited CVD TiC [2]. As is typical of laser-melted materials, a weak rippling was observed. Its period and amplitude were, respectively, about 10 and 0.5 pm. With the short pulses used the melt depth normally was about l p m . The coating melted in its entire thickness if the fluence was raised to 15 J cm -2, but then the modification was no longer homogeneous because of considerable plasma formation. In a better approach, the coating thickness was adjusted to the melt depth of lpm. The CO2 laser was employed to study the influence of (i) the wavelength and (ii) a longer irradiation time of about 0.1 ms which caused an increase in the melt depth down into the substrate. As can be seen in Fig. 2, a controlled modification was not possible. The coating was only partially melted. Locally, the coating was sometimes even removed, mostly because of melt expulsion and partly through mixing with the substrate. At the same time the substrate heated up considerably. The cobalt melted down to a depth of 50/~m and deep cracks formed which significantly reduced the tensile strength of the material. At this high temperature the cemented carbide decarbonized, and some cobalt may even have evaporated. Wherever the coating had not melted, any gas which developed could not escape and cavities up to 103 pm 3 formed below the interface. Figure 3 is a diffraction p a t t e r n of as-deposited TiC. The relative peak intensities deviate from the ASTM standard. The presence of (hhh) peaks which are too large and (h00) peaks which are too small is indicative of the

395

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Fig. 2. CO 2 laser-modified TiC coating on WC/Co sectioned across a scan line; the scan speed w a s 50 m min 1 and the fluence 1150 J cm 2: (a) optical and (b) s c a n n i n g electron m i c r o g r a p h s .

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highly textured morphology with the (111) lattice planes parallel to the surface. A particle size of 35 nm was derived from the peak widths using the Scherrer formula. After laser modification the peaks were found at the same positions indicating that neither crystallographic nor compositional changes had occurred. Any shift greater than 0.5 at.% in oxygen or nitrogen concentration near the surface was explicitly ruled out by the AES results. In contrast with the p a t t e r n s from beforehand, the relative peak intensities found after laser modification matched the ASTM standard. This means that epitaxial regrowth must have been suppressed during resolidification and an isotropic microstructure formed instead. From the observed peak broadening, proportional to (cos 0) -1, a decrease in particle size down to 25 nm was inferred. In conclusion, the laser remelting produced only slightly smaller particles whereas the grain size was significantly reduced which resulted in a highly refined microstructure as seen in Fig. 1. Whenever the coating was melted through its entire depth, the TiC peaks showed a shoulder on the high angle side (Fig. 3, inset (c)). The shoulder revealed the formation of a cubic W C - T i C solid solution with a concentration varying continuously from titanium-rich to tungsten-rich across the interface. The solution extended down to 5 pm as inferred from the EDX

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results, representing a metallurgical bonding between coating and substrate. This phase is indeed visible e.g. in Fig. 2(b) below the "drop". In Fig. 4 the friction coefficients of the original CVD and the lasermodified coatings are compared. At the beginning the modified coating had a lower friction because of its smoother surface. With increasing numbers of revolutions the micro-asperities in the original CVD coating were worn off and the friction decreased. With steel balls the friction coefficient remained above the value for the remelted coating and with TiC balls it fell slightly below. The wear of the balls seems to have been marginally higher with the laser-modified coating. However, no wear of the coating itself was observed.

4. D i s c u s s i o n The microroughness of TiC applied by CVD leads to spectrally selective absorptance [3]. As a consequence of the grain size being of the same order as the Nd: YAG laser wavelength, most of the energy was absorbed. An absorptance of above 90% was measured. This provides an excellent control in favour of a homogeneous modification. In contrast, the surface was essentially flat (i.e. metallic) to the 10.6 pm wavelength of the CO2 laser and most of the energy was reflected. Nevertheless, the grains acted as perturbations which accounts for the locally irregular absorption. Hot spots resulted which amplified the irregular behaviour even further by positive feedback. This explains the inhomogenous modification with the CO2 laser. The microcracks shown in Fig. l(b) were caused by thermal stress following the contraction of the resolidified TiC on the cold underlying material. During cooling a stress of approximately E ~ A T builds up. Allowing for stress relief down to half the melting point, as is usual for ceramic materials, the temperature range of stress accumulation AT is 1400 K. Taking a Young's modulus E of 440 GPa and an expansion coefficient ~ of 8 × 10 -8 yields a stress of 5G Pa . This exceeds the tensile strength of TiC which, although depending somewhat on the microstructure, can reasonably be assumed to be below 1 GPa. The crack formation cannot be prevented by

397 preheating the sample up to 900 K, as this still yields a stress above 1 GPa. At higher temperatures TiC oxidizes. The growth morphology of the original CVD layer and of the lasermodified TiC can be correlated with the solidification entropies AS [4]. For the CVD process AS is greater t han 100 J mo1-1 Z -1. At such a large value of AS growth rates are highest in the direction (111} and a faceted morphology is formed. This explains both the texture and the microroughness. For the solidification from the melt, AS is about 20 J mo1-1 K -~. This is of the order where (111} and (100} growth rates are similar and where a non-faceted morphology is favoured. After laser remelting there is indeed a homogeneous, equiaxed microstructure with a microscopically smooth (though atomically rough) surface.

5. Conclusion Good control of modification is guaranteed using a Nd: YAG laser since its energy is almost ideally absorbed by a TiC layer made using CVD. Owing to the high cooling rates obtained with short laser pulses epitaxial regrowth is prevented and the microstructure is refined. The microstructure is equiaxed, and the surface is microscopically smooth, owing to the relatively low entropy associated with laser remelting. The coating can be alloyed to the substrate by the formation of a W C - T i C phase of smoothly varying co n cen tr ation across the interface.

Acknowledgments The CVD coatings and the tribological measurements were made at CSEM. We are indebted to Dr. R. H a u e r t (EMPA) for the AES and to Professor H. P. Weber for helpful discussions. This work was funded by FSRM.

References 1 M. von Allmen,Laser-Beam Interaction with Materials, Springer, Berlin, 1987. 2 H. E. Hintermann, J. Vac. Sci. Technol. B, 2 (1984) 816. 3 A. A. M. T. van Heereveld, Surface roughness and spectral selectivity, Thesis, University of Gr6ningen, 1988. 4 W. Kurz and D. J. Fischer, Fundamentals of Solidifications, Trans. Tech., Switzerland, 3rd edn., 1989.