Field of the Invention This invention relates to wear resistant coatings . The invention has application to a broad range of abrasive environments , but has particular application to

environments that are abrasive and corrosive . Background

In the course of extracting valuable minerals from a mined ore, the ore goes through a number of different processing stages. In the case of some nickel-containing ore, the preferred processing route involves high pressure acid leaching (HPAL) in autoclaves.

The ore is ground to provide a particle size that is suitable for processing and is then formed into a slurry by the addition of recycled process water. The slurry is supplied to an autoclave where sulfuric acid is added. The conditions in the autoclave are controlled depending on the mineralogy of the ore feed to maximize nickel

leaching. However, processing conditions in the autoclave generally involve an elevated pressure in the range of 30 to 52 atm, temperatures in the range of 120°C to 270°C and acid addition of 200 to 500 kg/t of ore. Agitators are immersed in the hot acidic slurry to achieve suspension of solids .

In order to withstand these conditions, autoclaves are lined with titanium. The agitators are manufactured from titanium alloys but they are subject to considerable abrasion from contact with the ore. Accordingly, agitators are subject to very abrasive and corrosive conditions and are typically manufactured with a wear resistant coating to improve blade life. The HPAL process operations are continuous. Accordingly, maintenance on the autoclave and agitators requires the autoclave to be taken out of service. Typically, this involves shutting down the autoclave for a period of about 3 weeks , including bringing the acid down in temperature and pressure, de-scaling, routine corrosion and wear monitoring, changing over agitators and recommencing operations . Autoclaves are typically shut down every 9 months so that, amongst other factors, the wear of the agitators can be assessed. If the agitator blade has worn to an extent that agitator efficiency is adversely impacted, the agitator is replaced. If not, the agitator is placed back in service and wear is assessed again in a further 9 months.

Historically, agitators were not coated with a wear resistant coating. They were instead formed of Grade 5 or Grade 12 titanium.

Wear resistant coatings of titanium dioxide (Ti0 ₂ > were adopted subsequently to improve the service life of the agitators . The titanium dioxide coating is applied by thermal spraying of T1O ₂ particles directly onto an agitator. An example of a microstructure of a TiC> ₂ coating is shown in Figure 1. The coating provides good wear resistance and it can be applied on-site at the autoclave. However , achieving a good coating requires a high level of preparation work to the agitator surface to ensure that it is free of contaminants. Even then, the T1O ₂ coating forms a generally poor mechanical bond with the surface. Coating depth is limited to 0.5mm because it is not possible to build up multiple layers of the coating. Due to properties of the T1O ₂ coating, the coating must be totally removed from the agitator before a fresh coating is applied. An alternative wear resistant surface for agitators is reaction welded titanium nitride (typically a mixture of titanium and titanium/nitrogen intermetallics) . An example of a microstructure of a titanium nitride hard-facing surface is shown in Figure 2. This hard-facing is formed by producing a molten titanium weld pool in the agitator substrate and supplying nitrogen/argon gas mixture to the weld pool to cause a chemical reaction. As more nitrogen reacts with the titanium, the predominant phases produced change to higher nitrogen containing phases causing the coating to become brittle and porosity levels to increase. Due to the fact that this product is produced by an exothermic chemical reaction, and is limited by kinetic factors , the product is typically heterogeneous . The hardness of this product is not uniform since hardness is related to the diffusion of nitrogen, which occurs at slower rates farther from the surface .

As shown in Figure 2 , the microstructure is a mixture of various titanium nitride intermetallics and a solid solution containing both titanium and dissolved nitrogen. The titanium nitride intermetallics are hard and provide the reacted surface with good wear resistant properties. With titanium nitride hard-facing, the reaction depth is generally around 1.5mm. This process consumes the

component and the resultant reacted surface is

metallurgically bonded. While such bonding is beneficial for ensuring that the hard-facing remains on the agitator, the coating process involves consuming part of the agitator. This is problematic because it can change the tolerances of a product being coated. This can be critical to agitator efficiency. Furthermore, the hardness of the coating is off-set by an increase in brittleness that can lead to micro and macro cracking. Due to dilution of nitrogen into the titanium substrate to depths well below the visual reaction zone , titanium nitrided components are not typically re-nitrided because of the resultant reduction in mechanical properties of the base material .

There is a need for an improved wear resistant surface that is suitable for abrasive and corrosive conditions . It is advantageous for the surface to be able to be reapplied easily without damage to the component.

Summary of the Disclosure

The applicant has recognized that titanium carbide (TiC) has beneficial properties that make it suitable to form a wear resistant coating for abrasive and corrosive

environments. Specifically, TiC has a Vickers hardness of 2200 (which is harder than nickel-containing ore) and, importantly, it has a specific gravity that is similar to titanium. This means that TiC particles will not sink in a hopper containing titanium particles and titanium based alloys, and will not sink quickly in molten titanium alloys.

The applicant has further recognized that coatings of TiC can be formed by incorporating TiC generally in a solid state into a molten matrix material of titanium or titanium alloy. Effectively, solid TiC particles are embedded in a commercially pure titanium alloy matrix. The high hardness of the TiC particles imparts high wear resistance and the commercially pure titanium matrix imparts corrosion resistance and ductility.

Accordingly, the invention provides in one aspect a feed for forming a wear resistant coating on a substrate by a welding process that heats the feed and the substrate, the feed comprising:

(a) 35 to 50 wt% titanium carbide; and (b) a balance of commercially pure titanium or titanium alloy and incidental impurities .

The feed may comprise titanium carbide in the range of 35 to 45 wt% . Optionally, the feed may comprise titanium carbide in the range of 35 to 42 wt% .

The titanium carbide and the commercially pure titanium or titanium alloy may be in the form of particles .

The particle size of particles in the feed is limited by practical aspects of particle feeders. It is anticipated, however, that particles sizes up to 250μπι are suitable and, indeed, even larger particle sizes may be used in the feed. Particle size selection is subject to competing factors of small particles having poor flow properties and of small particles requiring less heat input to cause melting. For this reason, the titanium or titanium alloy particles may have a particle size that is less than the size of the titanium carbide particles. In one embodiment, the titanium carbide particles have a particle size in the range of 5 to 170μιη. The titanium or titanium alloy particles may have a size in the range of 20 to 170um. The small particle size means that considerably less energy is required to heat the titanium alloy particles to their melting point to form molten titanium alloy.

However , the TiC particles predominantly remain solid during the process , except for small particles which dissolve in the molten titanium alloy and precipitate as TiC upon cooling and a small proportion of carbon that remains dissolved in the matrix having a small hardening effect. A sufficient gas shield, typically argon, is provided during welding so that oxygen and other

contaminants do not affect the weld. Below 35 wt% titanium carbide particles , the volume of titanium carbide particles in the coating drops off to an extent that the commercially pure titanium alloy matrix becomes excessively exposed to the wear environment and, therefore, the wear resistance of the coating decreases. Additionally, there is an increase in the extent to which titanium carbide particles will dissolve in the

commercially pure titanium alloy matrix such that the carbon content of the matrix increases causing smaller secondary titanium carbides to from. These smaller titanium carbides are less beneficial to large particle abrasion but are useful for providing erosion from process fluid flow. Above 50 wt% titanium carbide particles, the coating becomes difficult to weld because the stresses created upon solidification are spread over a smaller volume of matrix formed by commercially pure titanium or titanium alloy. This increase in stress per volume leads to cracking of the overlay.

The feed may further comprise an inert conveying gas for entraining the particles and for providing an inert shield to a weld pool formed by the welding process before the weld pool solidifies. Optionally, the gas may be argon.

The titanium alloy particles may comprise alloying elements with the balance being at least 50% titanium and incidental impurities .

In accordance with another aspect, there is provided a method of forming a wear resistant coating, the method comprising the steps of : (a) delivering the feed according to the aspect described above to a surface of a substrate; (b) exposing the feed and the substrate to sufficient energy to cause at least the commercially pure titanium or titanium alloy particles in the feed to melt ; whereby the titanium carbide particles are embedded in the molten commercially pure titanium to form a wear resistant coating on the substrate. Step (a) may involve conveying the feed to the substrate in an inert conveying gas and controlling the flow of the conveying gas to control the feed rate of the feed.

The method may involve depositing one or more layers of the wear resistant coating on the substrate to build up the thickness of the wear resistant coating.

Titanium has a high affinity for oxygen and, as a result, a titanium alloy substrate will have an oxide surface layer. Many techniques for coating a titanium substrate involve removing the oxide layer (for example, such as grit blasting or baking the substrate) in an inert atmosphere . Another option involves removing the surface layer from the substrate by removing a contaminated surface layer from the substrate. Such removal may be by chipping the surface layer off the substrate. This may involve milling. Sufficient bonding is generated without oxide removal between coatings formed according to the method, but bonding is improved when the oxidized layer is removed. The method may further comprise carrying out steps (a) and (b) while the substrate is exposed to the ambient atmosphere . In other words , it is not a

requirement of the method to be carried out in an inert environment to avoid exposure of the coating and substrate to oxygen in the ambient atmosphere. The method may further comprise a step of pre-treating the substrate to remove contaminants. The pre-treating step may be carried out while the substrate is in contact with the ambient atmosphere .

The surface pretreatment step is selected to remove oxygen, iron and other contaminants from the surface. In one form, the pretreatment step involves chipping the substrate with a tungsten carbide burr to remove

contaminants. Such chipping may be performed in the presence of the ambient atmosphere , i.e. in the presence of oxygen .

Steps (a) and (b) may be provided by a welding technique. Such welding techniques include laser cladding, TIG welding, MIG welding and PTA welding.

In a further aspect, there is provided a wear resistant coating formed on a substrate, wherein the wear resistant coating comprises particles of titanium carbide dispersed in a matrix of commercially pure titanium or titanium alloy. The titanium alloy may comprise alloying elements with the balance being at least 50% titanium and

incidental impurities .

The coating may be metallurgically bonded to the

substrate. Additionally, such bonding may occur during application of the wear resistant coating on the

substrate .

The titanium carbide particle size may be in the range of 20 to 170μπι.

The titanium carbide particles may comprise 35 to 45 wt% of the wear resistant coating. Optionally, the titanium carbide particles may comprise 35 to 42 wt% of the wear resistant coating. The carbon content in the matrix may be in the range of 0 to 2 wt%. It has been shown by the applicant that a coating in accordance with the aspect described above has

significantly improved wear rates compared with titanium nitriding when tested using a dry sand rubber wheel method. These wear rates are derived from the

microstructure of the coating which comprises hard titanium carbide particles generally homogenously

dispersed in a matrix of titanium or titanium alloy. The hard titanium carbide particles resist wear and the matrix provides corrosion resistance.

It also has the benefit over titanium dioxide coatings that it is metallurgically bonded. It is expected to provide coated substrates with a longer service life due to the improved wear resistance rates. However, corrosion becomes an important consideration when the service life of a component is extended, such as in an autoclave that processes nickel-containing ore. It will be appreciated that the corrosion resistant properties of titanium will sustain the service condition of the coated substrate in the corrosive conditions.

The wear resistant coating may be formed to a thickness of greater than 0 to 2 mm. Optionally, the coating may be formed to a thickness of greater than 0 to 4 mm. Further optionally, the coating may be formed to a thickness of up to 10 mm.

Brief Description of the Drawings An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings , in which : Figure 1 is a cross-section showing the microstructure of a T1O ₂ wear resistant coating.

Figure 2 is a cross-section showing the microstructure of a TiN wear resistant coating.

Figure 3 is a cross-section showing an embodiment of a microstructure of a TiC wear resistant coating formed according to the invention.

Figure 4 is a perspective view of overlapping sections of the wear resistant coating in Figure 3 formed on the surface of a titanium substrate .

Figure 5 is a schematic representation of an apparatus for forming a wear resistant coating.

Detailed Description

The description that follows is in the context of applying a wear resistant coating to a substrate of titanium alloy. It is important to appreciate, however, that the wear resistant coating may be applied to other materials that can be directly welded with titanium, and other alloys by use of a suitable butter layer.

An apparatus 1 for forming a wear resistant coating on a substrate 10 is shown in Figure 5.

The apparatus 1 comprises a spray nozzle 20 having an elongate body. The spray nozzle 20 includes a laser generator 22 that generates a laser 40. The laser

generator 22 is aligned along a central longitudinal axis of the elongate body. A sleeve surrounds the laser generator 22 to form an annular feed flow chamber 24. The laser generator is linked to a power source 26 to generate the laser 40 with sufficient energy to melt small particles of titanium in the range of 20 to 170um. The chamber 24 is linked via a conduit to a reservoir 28 of feed particles for forming the wear resistant coating. The reservoir 28 is supplied with argon gas from a gas source 30 to fluidize the particles and convey the entrained particles through the conduit and chamber 24 and then onto the substrate 10.

The flow of particles and gas from the chamber 24 is controlled to converge from the annular opening

surrounding the laser generator 22 in a flow stream

(denoted by an arrow marked 50 in Figure 5) that

intersects the laser 40 at the surface of the substrate 10. Accordingly, the feed particles are subject to high temperatures at the surface of the substrate 10. The feed particles comprise a blend of titanium alloy particles and titanium carbide particles . The titanium carbide particles comprise 35 to 50 wt% of the blend. Both the titanium particles and the titanium carbide particles have a size in the range of 20 to 170 um.

It will be appreciated that alternative configurations for supplying feed particles to the surface of the substrate 10 may be adopted. For example, the titanium alloy particles and the titanium carbide particles may be supplied from separate reservoirs and combined together in the chamber 24 so that a blend of feed particles is formed in the chamber 24 and is supplied as described above to the surface of the substrate 10. Alternatively, the blend of particles may be formed at the surface of the substrate 10 by supplying the titanium alloy particles and the titanium carbide particles through separate nozzles that direct the particles to the point on the surface of the substrate 10 that is irradiated by the laser .

The applicant has observed that, although the laser melts the titanium alloy particles, the titanium carbide particles generally remain in a solid state and become embedded in the wear resistant coating by being surrounded in a matrix of titanium alloy.

The applicant has also observed that because the laser energy is selected to melt the titanium alloy particles only, a weld pool generated by the laser quenches so rapidly under the argon shield gas (powder gas) that oxygen is unable to react with the molten titanium. This results in a wear resistant coating that is generally free of oxygen .

One example of conditions used to prepare a wear resistant coating is outlined below.

Substrate : Titanium grade 5

Substrate thickness : >25 mm

Ti particles: Amperit 155.093

Ti particle size/density: 90 to 125 μπι / 4.51 g/cm ³

Ti particle weight% : 56

TiC particles: Titanium Carbide TK

TiC particle size/density: 45 to 90 μπι / 4.9 g/cm ³

TiC particle weight% : 44

Substrate pre-cleaning: acetone wash

Conveying gas and flow rate : Argon at 5 1/min

Ti/TiC particle feed rate: 16 g/min

Laser : Laserline LDF 6,000-100

Spot size: 8.5 mm

Heat input 30.59 J/mm ²

Laser travel speed: 750 mm/min

Spacing between laser passes : 3-4 mm An example of a microstructure for a wear resistant coating formed in accordance with these conditions is shown in Figure 3. Discrete particles of titanium carbide are shown dispersed generally homogenously in a generally continuous matrix of titanium alloy. The titanium alloy of the substrate is metallurgically bonded with the wear resistant coating. A wear resistant coating formed by a series of side-by-side laser passes is shown in Figure 4. The feed rate of particles identified above produces a wear resistant coating thickness of 1.6 mm. However, it is possible with this process to build up the thickness of the coating by running subsequent laser passes and feed particles over already formed coating. In this manner, it is possible to build up the coating to any desired depth, but it is expected that thicknesses of up to 10 mm will be suitable for a wide variety of applications. For example, the wear resistant coating may be applied to agitator blades for autoclaves, diffuser cones, wear plates and valve components .

Wear resistance of the coating was tested for comparison with other wear resistant coatings. The testing involved interposing dry sand between a wear resistant coating and a spinning rubber wheel. The dry sand acts as an abrasive which is driven across the surface of the wear resistant coating by the wheel. The results of the test work are shown in the Table 1 below.

Table 1 The titanium carbide wear resistant coating designates a coating formed in accordance with the above conditions . The coating had a mass loss that is approximately 14 times less than the titanium nitrided duplicate coating and even less than the untreated Grade 12 titanium. On the basis of this test work, the applicant believes that the Ti/TiC coating has good wear resistance properties. In the claims which follow and in the preceding

description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

It will be understood to persons skilled in the art of the invention that many modifications may be made without departing from the spirit and scope of the invention.

It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.