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Amorphous diamond coating of tungsten carbide and titanium carbonitride for erosive slurry pump component service

Abstract

Tungsten carbide spray coatings have become well established for resisting abrasion and erosion in pumps used in conventional oil production and in oil sands operations. To achieve additional benefits from the extreme wear resistance of tungsten carbide its use is being extended to solid forms for some critical components. Slightly harder titanium carbonitride-based cermets have much lower density and coefficient of thermal expansion and for these reasons are being considered as an alternative to tungsten carbide. PVD amorphous diamond coating also has potential to further increase the service life of selected pump parts fabricated from solid cermets. Micro-abrasion testing and single scratch and nano-indentation evaluation have been carried out on non-coated and PVD amorphous diamond coated WC-4.8% TaC–4.5% TiC/6% Co–1% Cr and TiCN-17% WC/8% Mo materials. Data obtained illustrate that the tungsten–carbide based product has superior wear properties to the titanium carbonitride material and that PVD amorphous diamond coating of both cermets enhanced wear resistance significantly and displayed potential for successful service application.

Introduction

Wear attack is a principal concern in the operation of slurry pumps. Material removal alters the dimensions and surface condition of integral components over time thereby influencing significantly their hydraulic and mechanical performance. This reduces pump efficiency, reliability and most importantly operating life. Abrasion and slurry erosion are the predominant wear mechanisms affecting high speed multistage pumps used for water injection in conventional oil production and during bitumen conversion in oil sands operations. Abrasion is caused mainly by solid particles in the slurry which are entrapped in narrow clearances between pump components in relative motion and low angle impingement, for example, sleeve and bushing combinations. Erosion occurs when material is lost from a surface through impact with solid particles in the transported slurry, for example, on impellers. The wear rate of a pump component is determined by the properties of both the component surface and the solid particles, and by the flow conditions of the slurry, most importantly velocity, impingement angles and solids concentration. From a contact mechanics perspective, the wear rate is dominated by plastic deformation of the component surface. This should be prevented or restricted in order to minimize the wear rate. Under certain operating conditions, this can be best achieved by applying hard material on the component surface. Generally, when the hardness of a component surface being abraded is increased to close to the hardness of the abrading material, the wear rate of the component can be greatly reduced. When the surface is about 30% harder than that of the abrasive, its wear rate is very small. The type of solids handled by slurry pumps depends on the operation. In water injection wells and bitumen conversion operations, the solids are predominantly silica-based with hardness values between 6 and 11 GPa. There is also the possible presence of fine alumna particles that are added as a well cleaning agent in conventional oil production or can originate as a constituent of the catalyst used during bitumen conversion in oil sands operations. Alumna has a hardness between 20 and 30 GPa and is the hardest constituent encountered in oil industry process slurries. Thermal sprayed tungsten carbide coatings with their high hardness (usually from 10 to 19 GPa) and moderate toughness, have become established for resisting wear of multistage high speed pump components used in oil production to transport fine solids laden fluids. Sintered tungsten carbides with lower binder contents are generally harder (usually from 14 to 27 GPa), denser and have higher cohesive strength and integrity compared to sprayed forms thus they have better abrasive and erosive wear resistance. Consequently, to achieve further performance benefits from the extreme wear resistance of tungsten carbide, its use is being extended to selected critical components which are amenable to production in solid sintered forms. The hardnesses of titanium carbonitride-based cermets are similar to tungsten carbides, but they offer much lower density and also have an important coefficient of thermal expansion advantage which makes them attractive possible alternatives to tungsten carbide. Commonly used sintered tungsten carbides and the more novel titanium carbonitride are among the hardest engineering materials, but they are slightly softer than alumna. To achieve further increases in wear resistance, coating the two cermets with material harder than alumna are required. Diamond or diamond like coatings are considered to offer the most potential. The purpose of this study is to compare the wear resistance and other mechanical properties of two selected sintered WC and TiCN cermets; to establish the benefits of coating the two cermets with tetrahedral amorphous carbon (ta-C) as a method of improving their overall wear resistance for high speed multistage pump components for transporting oil production slurries.

Tungsten carbide and titanium carbonitride base materials

The WC cermet contained 4.8% TaC, 4.5% TiC and had a 6% Co, 1% Cr binder content. This is a standard highly abrasion/erosion resistant commercial grade recommended for fluid/slurry handling parts. The TiCN cermet contained 17% WC–8% Mo. The WC sample was in the form of a hub ring component for a multistage pump, with a maximum diameter of 143 mm, flange thickness of 3 mm, hub diameter of 82 mm and length of 41 mm. The TiCN sample was a 5.3 mm thick disk with a 23.8 mm diameter.

Crystalline diamond coatings

These are usually made by chemical vapor deposition (CVD) and are extremely hard. Disadvantages are adherence to substrate is low; processing temperature is high (>600°C) and this induces thermal stresses in both coating and substrate thus increasing the tendency to adhesive failure or cracking.

Amorphous carbon coatings

These are deposited by a variety of methods. They have superior substrate adhesion and they can be processed at a lower temperature compared to CVD diamond coatings.

Process

TETRABOND tetrahedral amorphous carbon (ta-C) coatings were deposited at a temperature of 93°C, by Multi-Arc SC, on WC and TiCN cermets, using an enhanced arc physical vapor deposition (PVD) process. These coatings are applied typically at a thickness of 1–2 μm and processing temperatures between 20 and 150°C.

Hardness and elastic modulus of the coatings

These properties were measured by means of a depth sensing indentation technique using an ultra-micro indentation instrument with a diamond Berkovich indenter and a peak load of 20 mN. Resultant contact depths between indenter and coatings were less than 0.08 μm and were small enough to ensure that all the plastic deformation and most of the elastic deformation took place within the coatings. The hardness and elastic modulus of substrate

Coating adhesion

Scratch testing was chosen from many options to evaluate the bond between the coating and substrate, since it has been proved to be the most practical method for evaluating the adhesion of hard, thin coatings. Bond evaluation by scratch testing is based on the premise, that when the mean compressive stress over an area in a coating exceeds a critical value, the coating detaches from the substrate to lower the elastic energy stored in the coating.

Critical contact load and impact velocity for substrate yielding

For a coating to be effective in wear protection, the following three conditions are necessary. The overall compressive stress in the coating should be low so that large-scale coating detachment will not take place. To avoid interfacial crack development, plastic deformation caused by individual contacts must not take place at the coating/substrate interface. Adhesion of the coating to substrate is sufficient to withstand the cyclical interfacial stress generated during wear.

Test method

An evaluation was carried out to investigate: (1) the wear resistance of the coatings; (2) the wear resistance of the substrates; and (3) whether or not the adhesion of the coatings to substrates was sufficient to withstand the cyclical interfacial stress generated during the wear process. Because the coated systems are considered to be most suitable for resisting low-angle impingement erosion and/or low stress abrasion, the most appropriate wear properties assessment method should simulate such

Discussion

Current wear protection practice for multistage oil production pumps transporting solids bearing slurries, involves the use of high velocity oxy-fuel (HVOF) thermal spray coatings of WC applied to typically martensitic stainless steel base components. To extend the mean time between failures, the next development being pursued is the installation of selected solid tungsten carbide components with higher intrinsic wear resistance throughout their sections. Their low stress abrasion resistance

Conclusions

Solid sintered TiCN-based cermet exhibited inferior micro-abrasion properties compared to a WC grade recommended for fluid handling service. It was not considered to be a suitable alternative from a wear performance perspective. TETRABOND PVD amorphous diamond coating on the WC cermet exhibited very high hardness, good adhesion and much higher abrasion resistance than the base material. The coating offers an opportunity to enhance the wear performance of carbide pump components in low velocity