Elsevier

Wear

Volume 256, Issues 1–2, January 2004, Pages 81-87
Wear

Development and testing of HVOF-sprayed tungsten carbide coatings applied to moulds for concrete roof tiles

https://doi.org/10.1016/S0043-1648(03)00348-XGet rights and content

Abstract

Corrugated steel moulds, so-called ‘gliders’ used to shape, densify and burnish concrete roof tiles are subject to severe abrasive sliding wear caused by the action of hard quartz grains in the moving concrete mass. Hard surface coatings based on tungsten carbide (WC) applied to the gliders are thought to reduce substantially the friction between concrete slab and forming tool and hence the loss of tool material, and also to improve their burnishing (smoothing) capability. Various HVOF-sprayed WC-based coatings on hardened X210Cr12 tool steel substrates were evaluated with respect to their performance in abrasive wear tests. Three tests were performed that stressed the coating surfaces in different ways: a 2-body Taber® Abraser abrasive-rolling integral wear test, a 2-body dry sand/rubber wheel abrasion test according to ASTM G65-91, and a 3-body ring-on-block sliding wear test. Volumetric losses and changes in the surface roughness parallel and perpendicular to the direction of the movement of the concrete slab showed that coatings with a low surface roughness level but high roughness anisotropy, i.e. preferential ‘grooving’ parallel to the loading direction, performed best in ASTM G65-91 wear tests as well as in service tests under real operating conditions.

Introduction

The service life of industrial machinery is limited predominantly by mechanical wear and/or chemical corrosion. Increasing demand on productivity, efficiency, quality, and throughput of technical systems continuously increases performance-related stress on tools and machine parts. In Germany alone, wear and corrosion destroys annually the equivalent of about 4.5% of the GNP, corresponding to €35 billion [1]. Among a plethora of approaches to improve the mechanical performance and corrosion resistance of metals, coating by thermal spraying, such as atmospheric plasma spraying (APS) and high velocity oxyfuel (HVOF) flame spraying is one of the most versatile, economical and hence frequently utilised surface technologies [2], [3], [4].

In particular, HVOF spraying has developed into a reliable technique to apply hard, dense, tribologically superior, and well adhering metallic and composite cermet coatings to a great variety of metallic surfaces [5].

This contribution deals with attempts to improve, by application of HVOF-sprayed tungsten carbide (WC)-based coatings, the service life of moulds (gliders) used to shape concrete roof tiles (Fig. 1) [6]. The large proportion of quartz grains in the concrete paste subjects this mould (glider) to severe hard mineral grain sliding wear.

The performance profile of WC-based thermally sprayed coatings includes high indentation hardness in excess of 15 GPa, excellent resistance against abrasion, solid particle erosion (SPE), cavitation erosion, and chemical corrosion, low porosity, low friction coefficient as well as the option of powder processing by a broad spectrum of thermal spraying techniques [7]. These techniques include conventional flame spraying, HVOF spraying, atmospheric plasma spraying (APS), low pressure plasma spraying (LPPS) [8], and even underwater plasma spraying (UPS).

It is the aim of the present investigation to develop tungsten carbide coatings that will increase the service life of machine parts made from X210Cr12 steel and designed to burnish moist concrete slabs extruded into a corrugated forming piece (glider) to produce roof tiles. Observations of in-service performance of gliders coated with WC–Co10Cr4 show that with time a surface roughness develops that results in a combination of high frictional forces between glider and concrete, increasing power consumption, and reducing burnishing capability. Hence coatings must be optimised for low friction and superior burnishing performance. This can be achieved by either coatings with a generally low surface roughness profile or a roughness characterised by Rz values that are lower in the direction of movement of the concrete slab (Rz,pl) compared to the direction perpendicular to it (Rz,pr). Through experiments with different coating formulations and wear test protocols such optimised coatings should be developed. In particular, a suitably realistic wear test should be identified that could provide information on in-service performance of the tribological system coating–concrete–aggregate without the need of time-consuming and hence expensive praxis tests [9].

In this study eight WC-based coatings differing in their composition, powder particle size distribution, and production methods were applied by high velocity oxy-fuel spraying-diamond jet hybride (HVOF-DJH) to the surface of hardened X210Cr12 tool steel, and their performance tested under three different regimes of wear that were simulated by (i) a 2-body Taber® Abraser abrasive-rolling integral wear test, (ii) a 2-body dry sand/rubber wheel abrasion test according to ASTM G65-91, and (iii) a 3-body sliding wear test (ring-on-block test). A sintered hardmetal (WC–Co15, TRIBO Hartmetall GmbH, Immelborn, Germany) as well as uncoated hardened X210Cr12 steel were used as controls.

Section snippets

Materials and experiments

Test samples machined from hardened (annealed to HRC 59) X210Cr12 tool steel (orthogonal bars of 100 mm length, 100 mm width and 8 mm height for the Taber® Abraser test; orthogonal bars of 50 mm length, 25 mm width and 8 mm height for the ASTM G65-91 test; and parallelepipeds of 50 mm length, 35 mm width, 20 mm height, angle 45° for the 3-body sliding wear test) were coated by HVOF with the WC composite powders listed in Table 1. The Diamond Jet Hybride 2700 (DJH; Sulzer Metco) variant of HVOF spraying

Microstructures

Reflected light micrographs of cross-sections of the as-sprayed coatings of samples 1, 3, 5, and 7 are shown in Fig. 3. There are pronounced differences in homogeneity of the microstructure as well as porosity. Sample 5 with the highest proportion of the matrix metal cobalt appears to be very dense (0.4% porosity) whereas lower amounts of cobalt (sample 3) and addition of chromium (sample 7) leads to layered structures with rather high porosity (3.54 and 3.40% porosity, respectively). This is

Microstructure and volume loss of abraded coatings

The three tribological tests selected appear to stress the coating surfaces in a specific and very different way indicating that different wear mechanisms are active [15]. In general, the visible surface deterioration increases from the Taber® Abraser test to the dry sand/rubber wear abrasion test to the 3-body sliding wear test. Table 5 shows the volume losses in mm3/km. Only one sample of each coating type was measured. However, samples behaving similarly were grouped together and standard

Conclusion

Tool steel (X210Cr12) specimens were coated by HVOF(DJH) spraying with 200–250 μm thick coatings of WC–Co12, WC–Co17, WC–Co10Cr4 and WC–Cr3C2–Ni. The dominating abrasive wear mechanisms these coatings underwent during a 2-body Taber® Abraser and a 2-body dry sand/rubber wheel (ASTM G65-91) test was sliding wear with microchipping and microploughing. The very high forces exerted on the coating surfaces during a 3-body sliding wear (ring-on-block) test lead to pronounced surface deterioration

Acknowledgements

The authors are highly indebted to Dr. Klaus Meltke, Faculty of Mechanical Engineering, Process and Energy Technology, Technische Universität Bergakademie Freiberg, for conducting the 3-body sliding wear tests. Critical comments of two reviewers are highly appreciated.

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