Elsevier

Wear

Volume 260, Issues 7–8, 7 April 2006, Pages 838-846
Wear

Microstructure and sliding wear behaviour of Tribaloy T-800 coatings deposited by laser cladding

https://doi.org/10.1016/j.wear.2005.04.020Get rights and content

Abstract

This work has focused on the obtainment of Tribaloy T-800 coatings by laser cladding on plane 18/8 stainless steel specimens (AISI 304). The appropriate selection of cladding parameters allowed defect-free coatings to be obtained with minimal dilution. In order to evaluate their microstructure, cross-sections of the coatings were examined by optical microscopy and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS). The elemental composition of the coatings was determined using an optical emission spectrometer with an excitation source (GDOES) and phase analysis was performed by X-ray diffraction (XRD). Several zones can be distinguished in the microstructure of the clad layer: a planar crystallization region at the interface with the substrate, followed by cellular and dendrite crystallization from the interface to the surface of the laser track and an overlap zone between tracks which is characterised by the coarsening of the structure and the formation of a lamellar eutectic phase. The mechanical properties were evaluated by hardness measurements and sliding wear tests (ball-on-disk and block-on-ring configurations) at room temperature and without lubrication. It was observed the great hardness (close to 850 HV0.3) achieved for the Tribaloy 800 laser coatings, which presented a wear coefficient (k) between one and two orders of magnitude lower than the substrate. The analysis of the clad worn surfaces showed that there was a transition from an adhesive-oxidational mechanism to a more severe plastic deformation and crack formation wear process with increasing the applied load.

Introduction

Laser cladding uses a laser beam to melt different alloying elements on a metallic substrate to form a coating with minimal dilution, free of pores and cracks, and perfectly joined to the substrate. This process can be used to coat large areas by overlapping individual tracks, but its capacity to protect smaller localised areas is what makes it unique. This capacity, together with the wide range of materials that can be deposited, allows the precise elaboration of surface properties for local service requirements at a relatively low cost [1]. Laser cladding by powder injection represents a valuable tool for protecting materials against wear, corrosion and oxidation, the deposition of thermal barriers, and for the repair of components and tools. Hardening with laser technologies has developed very quickly in recent years and has encountered numerous applications in industry; from plastic injection moulds and automobile parts [2], [3] to steam turbine components [4]. The repairing of these components is an emerging market for laser cladding.

Of the different materials used in aggressive environments, cobalt-base alloys stand out for their good physical properties and their corrosion and wear resistance. The composition of these alloys varies according to their application; their main alloying elements are Cr, W and Mo and to a lesser extent C, Fe, Ni, Si and Mn, among others. One alloy that is particularly useful in wear applications is Tribaloy 800 (T-800), which belongs to the family of Co-base alloys developed by DuPont in the early 1970s, in which Mo and Si are added at levels that exceed their solubility limit in order to induce the precipitation of a hard and corrosion resistant phase known as the Laves phase. This phase is very abundant in the alloy (35–70 vol.%) and its presence governs all the material properties. In particular, the Laves phase confers high resistance to abrasive and adhesive wear, but limits the ductility of the material and its impact strength [5], [6]. This makes it difficult to obtain crack-free coatings on small components.

The aim of this work is to study the microstructure and properties of Tribaloy T-800 layers deposited by laser cladding, as well as to determine the influence of these properties on the wear behaviour of them. Moreover, a study of sliding wear of the coating as a function of the applied load was performed to analyse if a transition in the wear mode occurs when a certain level of load is exceeded.

Section snippets

Experimental

The base material used was a stainless steel (AISI 304) with a standard composition of 18% Cr and 8% Ni and dimensions of 50 mm × 25 mm × 10 mm. The chemical composition of the initial Tribaloy powder is shown in Table 1, with an average particle size of 45 μm and a spherical grain shape. Cladding was performed using a CO2 laser operating at a power of 1800 W. The cladding parameters were modified to obtain optimum process conditions, establishing a power density on the piece of 200 W/mm2, a beam

Microstructure and hardness of clad layers

Fig. 2 shows a cross-section of one of the Tribaloy T-800 coatings. The layers, with an average thickness of 1.8 mm, present a uniform appearance, and no lack of adherence or presence of defects is appreciated.

The scanning electron microscopy study (Fig. 3) allows several regions to be distinguished in the microstructure of the clad layer: the track center zone, the overlap zone between tracks and the zone close to the substrate. As well, the higher magnification of the SEM makes possible to

Conclusions

The appropriate selection of laser cladding parameters allows high quality Tribaloy T-800 coatings to be obtained on an AISI 304 stainless steel substrate. The layers obtained are defect-free and are metallurgically joined to the substrate. A planar crystallization region is observed at the interface with the substrate, followed by cellular and dendritic crystallization from the interface to the surface of the laser track. The overlap zones between tracks are characterised by the coarsening of

Acknowledgement

The authors thank CICYT for financing Project MAT2001-3528-C03-02, under the auspices of which this study was undertaken.

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