Ceramic coatings - a growing technology

TWI Bulletin, March/April 1992

Andy Sturgeon joined TWI at the end of 1990 as a Senior Research Engineer in the Ceramics and Precision Processes Department. After obtaining a degree in physics, he began his research career at the Centre for Advanced Materials Technology at Warwick University, where he gained a doctorate for his work on bonding mechanisms between glasses and ceramics to metals. After this he spent two years with the ceramics group at Rolls-Royce plc, where he was involved with projects aimed at introducing advanced ceramic components into aero-engines. This was followed by a further two years with Alcan International Ltd, where he worked in developing new and novel processing routes to ceramic and ceramic composite materials, in particular sol-gel and chemically bonded ceramics (for example high strength cements and reaction formed ceramics).

His activities at TWI involve leading a number of projects on ceramic coatings and ceramic to metal bonding. Most recently he has been investigating HVOF thermal spraying as a process for putting down high quality ceramic coatings for wear, corrosion and electronic applications.

Andy Sturgeon reviews coating techniques, introduces the relevant ceramic materials and describes the capabilities of various spraying processes for depositing ceramics.

Ceramic coatings, like their monolithic counterparts, are used for their excellent resistance to extreme temperature, corrosion and wear, and are being used increasingly to modify the surface properties of components. The intrinsic properties of ceramics confer advantages in a wide range of applications. For example, they provide thermal barriers in aero-engine gas turbines, hard coatings to increase the life of cutting tools, and wear resistant coatings on rolls for use in the paper and steel industries.

They are also being considered for other application areas such as the electronics and biomedical fields, where the electrical or chemical nature of the coating is essential to the functioning of a component. Ceramic coatings offer distinct advantages when compared with bulk ceramics, in particular with regard to manufacturing cost, non-catastrophic failure and minimal changes necessary in physical design. For these reasons they are likely to be introduced into a number of applications long before structural ceramic components.

TWI has recently expanded its ceramic coatings activity by installing an advanced thermal spraying system based on the high velocity oxy-fuel (HVOF) process ( Fig.1). This coating process is at the forefront of today's developments aimed at improving coating quality and performance. To establish the full potential and limitations of this new coating process, TWI is undertaking a number of key projects in collaboration with its Industrial Members.

Coating techniques

The vapour phase processes, such as those based on the various PVD and CVD techniques, are regarded as thin coating processes. They rely on atom by atom deposition from a gas phase, which is necessarily slow, and as a consequence economically attainable thicknesses rarely exceed 100µm. On the other hand, thermal spraying techniques deposit a heated powder, at high velocity, and build up a thick coating as individual particles deform and adhere on impact. This is regarded as a thick coating technique, producing coatings from a few tens of microns to several millimetres.

Numerous other techniques fall into the category of wet processing, and one of particular interest is sol-gel. This 'wet' technique offers a route to thin ceramic coatings using colloidal or complex organic solutions as ceramic precursors.

As well as a difference in coating thickness, each of the above techniques is suited to deposition of different ceramic materials. In general, thermal spraying and sol-gel are the preferred deposition techniques for oxide ceramics, while PVD and CVD are preferred for non-oxide ceramics like nitrides and carbides. The wide range of ceramic coatings prepared by the various techniques is illustrated by the examples given in Table 1.

Table 1 Ceramic coating types

Coating materials

Ceramic coatings include a very wide range of materials, but the majority can be divided into four groups depending upon their chemistry, namely oxides, carbides, nitrides and borides.

Non-oxide ceramic coatings

The carbides, nitrides and borides provide a range of high hardness coatings that have been used for many years in wear and abrasion applications, for example TiN and TiC are routinely deposited by the vapour phase processes of CVD and PVD. However, their very high melting point and tendency to dissociate or oxidise readily at moderate temperatures mean that these materials cannot be deposited by thermal spraying unless a suitable binding phase, usually a metal, is present. It should be noted that a whole range of coatings known as metal-bonded carbides exists, ^[4] such as tungsten carbide or chromium carbide in a metal matrix of, for example, cobalt or nichrome. These are a separate class of coatings designed primarily for various wear applications and are not discussed here, although they are important and compete with ceramic coatings in wear applications.

Oxide ceramic coatings

Oxide ceramics are of most interest as thermally sprayed coatings. Their relatively low melting points enable a number of single and mixed oxide compounds, including alumina, titania, alumina titania, chromia, and zirconia to be deposited by a range of thermal spraying processes. They exhibit a high hardness, although not as high as most carbides and nitrides, a high melting point compared with most metals, and good corrosion resistance. Some of the primary assets of oxide ceramic coatings are their stability at elevated temperatures and resistance to wear, particularly low stress scratching and polishing abrasion. The following ceramic coating materials are among those most widely encountered:

Alumina: offers good chemical stability and high hardness, which is retained at elevated temperatures. It has found use in various wear applications, particularly where low stress abrasion is encountered, It also has a high dielectric strength at room temperature and is commonly used to provide an electrical insulation barrier.

Alumina-titania: compositions of 3 to 40% TiO ₂ are used to provide dense, hard coatings that resist wear and corrosion. The lower melting point of the titania component results in an improvement in coating adhesion and density, although for higher percentages this is offset by a reduction in temperature capability.

Chromia: is also widely used as an abrasion resistant coating, often considered as giving the best performance of all the oxide ceramic coatings. It also provides excellent resistance to corrosion by acids and caustics,

Zirconia: has low thermal conductivity and exceptional temperature stability, and is widely used as a thermal barrier coating. It is usually used in a form which has been stabilised by addition of 5 to 20% of calcium oxide or yttria.

Thermally sprayed coatings

Thermal spraying is a generic name for a family of thick overlay processes in which the material, usually in powder form, is rapidly heated in a gaseous medium and projected at high velocity on to the substrate or component surface. Several methods are now available which are suitable for deposition of ceramic coatings. These are: flame spraying, air plasma spraying (APS), vacuum plasma spraying (VPS), detonation flame spraying (D-GUN), and recently high velocity oxy-fuel (HVOF) spraying, The important characteristics of each thermal spraying process are compared in Table 2.

Table 2 Thermal spraying processes

Flame spraying

This process ^[5] uses a flame to heat the coating material in rod or powder form. An oxy-acetylene flame having a temperature of 3300°C is used, into which ceramic material is fed, As shown in Fig.3, compressed air is passed through the nozzle, accelerating powder ceramic material on to the substrate at a velocity of up to 200 m/sec. Because of the relatively low temperature and low spraying velocities, only lower melting point oxide ceramics can be deposited.

The coatings are generally poor quality, having higher porosity, lower cohesive strength and lower adhesion than those applied using other processes. Despite its disadvantages, the process has become an established method for spraying oxide ceramic coatings and offers some advantages with respect to low cost and high manoeuvrability.

It is, in fact, one of the most widely used ceramic coating processes and possibly exceeds all others in terms of quantity of material deposited. A number of commercial variants are well established, for example the Rokide process of Norton Corp uses sintered ceramic rods to deposit coatings of alumina, chromia, zirconium silicate and magnesium aluminate.

Plasma arc spraying

In plasma arc spraying, ^[6] the thermal energy of an electric arc and its associated plasma jet provide the heat source to melt and project material at high velocity on to a surface. A schematic section of a plasma spray gun is shown in Fig.4. In this process a DC arc is struck between a central tungsten electrode in the torch and a water cooled copper nozzle which forms the anode. A flow of inert gases is passed through this arc where it forms a high temperature plasma stream from the nozzle. The spraying material, in powder form, is fed into the plasma flame by a carrier gas, where it is melted and propelled out of the gun.

The temperature of the plasma flame can reach up to 16000°C and is capable of melting even the most refractory ceramics. For standard power guns, typically 40kW, powder particle velocities of 120-400 m/sec are obtained, while for the higher power guns of up to 80kW becoming available, average particle velocities are over 600 m/sec. The higher temperatures and velocities compared with flame spraying result in improved coating adhesion and higher density. This process is used for deposition of oxide ceramics, such as alumina, alumina-titania, and chromia for wear applications. Some properties of plasma sprayed ceramic coatings are compared in Table 3. ^[7] The comparison of wear is shown for sliding wear against stainless steel and should not be considered as absolute. Plasma spraying has found particular success in depositing coatings of zirconia because of the very high temperature of the plasma flame. Plasma sprayed coatings of yttria stabilised zirconia, and zirconia based compounds such as calcium zirconate, are now routinely used as thermal barrier coatings in aircraft combustors and internal combustion engines.

Table 3 Properties of plasma sprayed coatings of oxides

Low pressure plasma spraying

In addition, the velocity of the plasma jet can be increased up to 900 m/sec in the low pressure chamber, thereby increasing the kinetic energy of the particles. The result is an improvement in coating adhesion and density. These advantages are, however, offset by the high equipment cost and requirement for an enclosed environment. Low pressure plasma spraying has found applications in certain chemical environments that require impermeable coatings, usually of zirconia or magnesium zirconate.

Detonation flame spraying

Detonation spraying ^[9] is somewhat similar to oxyacetylene powder flame spraying described above and was initially developed by the Union Carbide Corporation. In detonation spraying, measured quantities of oxygen, acetylene, and ceramic powder are metered into a firing chamber, as shown in Fig.5, and detonated by a timed spark. The resulting detonation creates a hot, high speed gas stream of about 4500°C and 800 m/sec, which instantly heats the particles and propels them at high velocity on to the substrate surface.

After the powder has left the barrel, a pulse of nitrogen gas purges it, and the whole process is repeated about four to eight times a second. The process is most suited to lower melting point oxide ceramics such as alumina, chromia, and alumina-titania. The high kinetic energy of the particles is converted to additional heat on impact with the surface and results in hardness, density, and bond strength higher than can be achieved by conventional plasma and flame spraying. Some properties of Union Carbide plasma sprayed and detonation flame sprayed coatings are shown in Table 4. The detonation process is commercially available through sub-contract facilities at various sites throughout the world.

Table 4 Properties of selected D-Gun ^TM and plasma sprayed coatings

*Determined with ASTM method C633-69. Bond strength varies somewhat with substrate composition, and the values shown are for either steel or aluminium.

High velocity oxyfuel

High velocity oxyfuel (HVOF) is one of the most significant developments in thermal spray technology in recent years. ^[10] The process was initially developed in 1982 by the Browning Engineering Co who developed the Jet-Kote HVOF spray system, the technology rights of which are now owned by Deloro Stellite. Today, several variants of the process are commercially available and include the Diamond Jet (Metco PerkinElmer), CDS (Plasma Technik) and Top Gun (Miller Thermal). The HVOF process uses an internal combustion jet to generate hypersonic gas velocities of 1800 m/sec, more than five times the speed of sound.

Suitable combustion fuel gases include hydrogen, propylene, propane, MAPP, and for the Top Gun, acetylene. The fuel gas is mixed with oxygen in a combustion chamber, as shown in Fig.6, where the gases are burnt to produce temperatures over 2760°C.

Powder is fed into the chamber using an inert carrier gas, such as argon or nitrogen, where rapid heating and acceleration of the powder particles takes place. The powder is propelled from the gun nozzle by the exiting gas and reaches velocities of up to 800 m/sec.

These very high particle velocities are similar to those achieved by detonation flame spraying.

Although the HVOF process has already been used to deposit high quality coatings of metal-bonded carbides, which are comparable in performance with coatings prepared by the detonation process, HVOF spraying of ceramics is still in its infancy. It is a process with considerable potential to deposit high quality coatings of the lower melting point oxide ceramics such as alumina, alumina-titania, and chromia. Compared with conventional flame and plasma sprayed coatings, improvements are expected in hardness, density and adhesion because of the much higher velocity achieved.

Applications

Today the largest applications for ceramic coatings lie in aero-engine gas turbines and cutting tool inserts, which together account for 85% of the market value. ^[11,12] This market is almost certain to expand over the next few years, with land-based engines (diesel and turbine), wear resistant parts, and heat exchanger applications identified as key growth areas. In terms of value of the coatings alone the world market for ceramic coatings in structural applications is predicted to grow at a rate of 12% annually to reach a figure of $3.3 billion by 1995. ^[11,12]

Wear and thermal barrier coatings

Deposition of oxide ceramic coatings by thermal spraying is an important part of this market, particularly with respect to thermal barrier coatings based on zirconia and zirconia containing compounds, and wear coatings of alumina, alumina-titania and chromia. HVOF spraying, along with detonation flame spraying, will have a key part to play in depositing high quality coatings of the lower melting point oxide ceramics, with plasma spraying remaining the most suitable technique for coatings of the more refractory oxides like zirconia.

Ceramic superconductor coatings

Newer applications are also likely to grow in importance, and two which have recently received a lot of publicity are high temperature ceramic superconducting coatings and bioceramic coatings.

Superconducting coatings ^[13] based on oxide compositions of, for example, yttrium-barium-copper oxide and bismuth-strontium-calcium-copper oxide are capable of remaining superconducting at temperatures above 77K, allowing for cheap and simple cooling of any superconducting device using liquid nitrogen. Such materials have been put down on to metallic and ceramic substrates by plasma and HVOF spraying for applications such as microelectronics packaging and magnetic shielding.

Bioceramic coatings

Bioceramic materials ^[14] are being used as coatings on porous metal surfaces for fixation of orthopaedic prostheses. Typical materials are based on calcium phosphates, such as hydroxyapatite, that mimic the mineral phase in bone and have the important property of being compatible with the surrounding tissue. To date these coatings have largely been deposited using plasma spraying equipment.

Superconducting ceramic and bioceramic materials are very sensitive to thermal history and spraying environment. HVOF is seen to offer a spraying process with, perhaps, more control over the spraying environment than is possible with plasma spraying leading to an improvement in coating performance.

Conclusions

• Ceramic coatings are prepared using a wide variety of deposition techniques, with each technique suited to a particular ceramic type and coating thickness.
• Vapour deposition processes, such as PVD and CVD, are mostly used to prepare thin ceramic coatings of oxides, carbides, nitrides and borides up to 100µm thickness.
• Thermal spraying processes - flame spraying, plasma spraying, detonation flame spraying and high velocity oxy-fuel (HVOF) - are regarded as thick coating processes, most suited to deposition of oxide ceramics like alumina, alumina-titania, chromia and zirconia.
• High velocity oxy-fuel is a new thermal spraying technique with potential to spray high quality coatings of lower melting point oxide ceramics, using commercially readily available equipment.

References