LASER PROCESSING OF METAL-CERAMIC COMPOSITE MULTILAYERS

Transcription

1 LASER PROCESSING OF METAL-CERAMIC COMPOSITE MULTILAYERS A. Schüssler, K.-H. Zum Gahr To cite this version: A. SchÜssler, K.-H. Zum Gahr. LASER PROCESSING OF METAL-CERAMIC COMPOS- ITE MULTILAYERS. Journal de Physique IV Colloque, 1991, 01 (C7), pp.c7-121-c < /jp4: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1991 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE IV Colloque (37, suppl6ment au Journal de Physique 111, Vol. 1, dkembre 1991 LASER PROCESSING OF METAGCERAMIC COMPOSITE MULTILAYERS A. SC~SSLER and K.-H. ZUM GAHR Kemforschungszentrum Karlsmhe GmbH, Institut fiir MateriaIfoschung I, Karhruhe, Germany Abstract Particulate metal-ceramic composite layers were formed using a CO2-Laser. Opposed to other methods hard phases (Tic, TiN, NbC) were embedded in a steel surface by liquid metal penetration into a predeposited layer of fine hard particle powders. The structure of surfaces processed by this method differs from those of conventionally treated surfaces. Hard phases incorporated in the steel matrix variied gradually in type, size and volume fraction from the surface to the substrate. Produced layers were in the range between 30 and 150 pm and free of pores and cracks. Introduction In the last years new layer concepts have been developed to ensure improved demands on structural components under complex working conditions. A number of techniques including powder metallurgy (1-3), plasma spraying (4),and PVD (5) had been employed to produce multi-layers, multiphase-layers and gradient-layers. Wear behaviour of metallic materials can be markedly changed by embedding hard ceramic particles. These second phases can affect wear by hardening of the matrix and/or reducing the real area of contact between a solid body and a counterbody. Incorporation of hard particles by C02-lasers provides excellent layer bonding and a limited heat input in contrast to other techniques. There are two different methods to insert particles into a substrate surface using a laser. Particles can either be preplaced on the substrate and lasered in a second step (two step process), or directly injected into a laser-generated melt pool (one step process). The method conventionally used to form composite surfaces is the laser melt-particle injection process (see e.g. 6,7). Particles are distributed in the entire melt bath due to excessive melt pool convection. However, this makes it difficult to establish a gradient or multilayer structure within the laser treated surface. In the present study melt pool convection attributed to surface tension forces had been avoided by using a two step process. Opposed to conventionally methods hard phases were embedded in a steel surface by liquid metal penetration into a predeposited layer of fine hard particle powders. Hard phases incorporated in the steel matrix variied gradually in type, size and volume fraction from the surface to the substrate. Article published online by EDP Sciences and available at

3 JOURNAL DE PHYSIQUE IV Experimental Methods A tool steel 90MnCrV8 (containing 0.9% C) was used as substrate. Powder mixtures of hard particles (Tic, TiN, NbC) and iron were applied as alcoholic suspensions onto the substrate prior to laser processing. Laser treatment was carried out using a 3.5 kw continuous wave CO~laser with a beam integrator for distributing laser energy. The specimens were laser treated in strips of 8 mm width protected by an argon gas shield. Power densities (7 k~/cm2) and scanning speeds (100 mm/min) were optimized to melt the substrate enabling the liquid steel (about Tm = 1420 OC) to penetrate between the hard phases (TiN, Tm = 2950 OC). The process parameters used avoid surface tension forces and consequently swirling-in of particles into the melt pool (8). By repeating the treatment procedure (coating and lasering) multilayer structures had been achieved. Results Microstructure of Com~osite Lavers In contrast to surfaces treated by conventional methods of particle incorporation the method used results in a distinct demarcation between particulate composite and the underlaying melt pool (fig. 1). Thickness of the composite layer is uniform and can be precisely adjusted by the thickness of the powder coating. Variation of processing parameters (laser power, scanning speed, laser optic) do not affect layer thickness. Single composite layers were in the range between 20 and 100 pm wherein particles are very homogeneously distributed in the martensitic steel matrix. Low overheating and bath convection of the method described result in an only small dissolution of Tic-particles, while other authors (see e.g. 9) report an entire dissolution using other processes. Fig. 1: Cross-section of a single laser track Imm f -' ~ Multilavers Multilayer structures of up to 5 single layers had been produced up to a total thickness of about 200 pm. Figure 2 shows a composite surface consisting of 3 single layers.. The sharp demarcation between these layers proves that no intermixing of particles occurred during laser treatment. An application of a double layer structure Tic - TiN is demonstrated in figure 3. The underlying TiN-particles act as a barrier for the dissolution of Tic.

4 Fig. 2: Three layer composite of NbC, Tic and TiN from the surface to the substrate Fig. 3a: TiN-particles on Tic-particles

5 C7-124 JOURNAL DE PHYSIQUE IV Fig. 3: Double layer composites on a steel substrate Fig. 3b: Tic-particles on TiN-particles Gradient lavers A hard phase gradient within the composite layer had been achieved by using coatings with varying iron content. Figure 4 shows a composite surface in which the volume fraction of the hard phase is stepwise raised from 33 vol.% TiN at the layer/substrate interface to 67 vol.% at the surface. Fig. 4a: Cross-section

6 Fig. 4b: Diagonal-section Fig. 4: Composite layer with varying volume fraction of TiN -particles from the substrate to surface the The step like change in the TIN content creates a gradual change of material properties within the composite and ensures improved wear resistance due to high content of hard phases at the surface. Generally crack initiation in the composite layer occurs by cracking of the particlelmatrix interface. Since the volume fraction of hard phases increases towards the surface and the particle spacing is reduced simultaneously, fracture toughness of the layer decreases in that direction. Fig. 5: Fracture micrograph of a gradient layer with varying volume fraction of TiN-particles on a steel substrate

7 C7-126 JOURNAL DE PHYSIQUE tv A fracture surface resulting from impact testing (figure 5) shows that crack propagation along the interface between the 67 vol.% and 44 vol.% TiN- layer is promoted compared to the layerlsubstrate interface. While incorporation of larger hard phases is more beneficial for increasing abrasive wear resistance, the value of residual stresses also increases with growing particle size. This results in an enhanced tendency to cracking. Residual tensile stresses can be reduced by applying a gradient in particle size. In figure 6 particle size of TiN-particles had been variied in three layers from 1.7 pm to 10.3 pm from the substrate interface to the surface. Fig. 6: Composite layer with increasing size of TiN-particles from the substrate to the surface Refetences 1. H. Berns, A. Fischer and Ch. v. Nguyen, "Wear Resistant Surfaces with a Hardphase Gradient", Proc. Int. Conf. on Surface Engineering, Toronto, Canada, 1990, pp A.R. Begg, C.W. Brown, N.E. Charman, "Graded Structure Composites", Britisch Petroleum, EP , M.Niino, A. Suzuki, I. Nirai, "A method of Producing a Functionally Gradient Material", National Aerospace Laboratory (Japan), EP , Y. Tsunekawa, H. Harada, M. Ukumiya, I. Niimi, "Heat Transfer in Thermal Barrier Coatings with Gradient Constituents Fabricated by Low Pressure Plasma Spraying", J. Japan Institute of Metals, 54 (1990), No.11, pp R. Fella, H.Holleck, H. Schulz, "Preparation and Properties of WGTiGTiN Gradient Coatings", Surface and Coatings Technology, 36 (1988), pp J.D. Ayers, T.R. Tucker, "Particulate Tic-hardened steel surfaces by laser melt injection", Thin Solid Films, 73 (1980), pp K.P. Cooper, "Improving the wear resistance by forming hard metal matrix-ceramic composite surface layers", Joum. Vac. Sci. Technol. A4 (1986), pp A. SchiiBler, K.H. Zum Gahr, "Incorporation of TiClTiN hard particles into steel surfaces using laser radiation", Proc. ECLAT, Erlangen 1990, pp T.H. Kim, M.G. Suk, B.S. Park, K.H. Suh, "The formation of surface-alloyed layers on carbon steel with high temperature materials (W, WC, Tic) by CO~laser and the effect of cobalt addition",optoelectron. Magaz., 4 (1988), pp