Alumina coatings - HVOF using hydrogen

TWI Bulletin, November/December 1994

Andy Sturgeon joined TWI at the end of 1990 as a Principal Research Engineer in the Advanced Materials and 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).

He has responsibility for a number of projects on ceramic coatings and surface engineering in general. Most recently he has been investigating HVOF thermal spraying as a process for putting down high quality coatings for wear, corrosion and electronic applications.

Using hydrogen as the fuel gas in HVOF spraying produces high quality coatings of alumina with better wear performance than air plasma sprayed alumina. Andy Sturgeon reports.

Thermal sprayed ceramic coatings offer an attractive and cost-effective solution for many industrial wear problems. Their excellent resistance to extreme temperature, corrosion and wear is used to modify the surface properties of many components in a wide range of applications as shown in Table 1.

Table 1 Current applications for thermal sprayed alumina and alumina-titania coatings

For example, they provide thermal barriers in aero-engine gas turbines and hard coatings to increase the life of, pump parts and rolls used in the petrochemical, paper and steel industries. They are also considered for other application areas in the electronics and biomedical industries, where the electrical or chemical nature of the coating is essential to the function of a component.

Today the largest applications for thermal sprayed ceramic coatings are zirconia thermal barriers for aero-engine gas turbines and to a lesser extent for automotive, diesel and land-based turbines. Thermal sprayed alumina coatings do not form part of this application area, but are used primarily for wear parts and other industrial applications, such as electrical insulation. They are used when resistance to sliding wear or abrasion is needed. Along with chromia, alumina and alumina-titania are preferred coatings for most general wear problems when damage by impact is absent.

The traditional thermal spraying processes for preparing high quality ceramic coatings are air plasma spraying, low pressure plasma spraying and detonation flame spraying. The high velocity oxyfuel (HVOF) process is an emerging thermal spraying process which is becoming established as a very competitive technique for depositing high quality metal bonded carbide coatings, like tungsten carbide-cobalt. In early 1991 TWI installed an HVOF system and recent work has indicated that the HVOF process is also capable of depositing high quality alumina based ceramic coatings.

High velocity oxyfuel spraying

An internal combustion process rapidly heats and accelerates a powder consumable to high velocities. The combination of a high chamber pressure of over 4 bar and gas flow rates of several hundred litres per minute generate hypersonic gas velocities of typically 1800 m/sec, and combustion temperatures above 2800°C. Some features of the HVOF process are compared in Table 2 with those of competing thermal spraying processes.

Table 2 Thermal spraying processes

Suitable combustion fuel gases include propylene, propane, hydrogen, MAPP and for some systems acetylene. The velocity and temperature reached by the particles are a complex function of both combustion conditions and parameter settings. Recent work has measured the velocity of HVOF sprayed alumina particles having a size of 5-15µm to be 600-1000 m/sec.

For a typical spraying distance of 150mm this gives a dwell time for the particles in the combustion jet of only 1.5 x 10 ^-4 to 2.5 X 10 ^-4 sec. In this short time the powder is rapidly heated by the surrounding combustion environment. For hydrogen fuel burnt with oxygen, a flame temperature of 2840°C is possible. With an acetylene flame, temperatures are higher and may be up to 3160°C. The temperature reached by the particles during their rapid passage through the gun and on to the moment of impact will depend on the particular HVOF equipment, fuel gas and spraying conditions used.

The lower temperature of the HVOF process relative to plasma and detonation spraying has created uncertainty over its ability to spray ceramic materials. The work reported here demonstrates that hydrogen may be used as a fuel gas to prepare alumina coatings of high quality.

Two fuel gases were used; acetylene and hydrogen. The powder sprayed was a 99.5% purity alpha alumina having a mean particle size of l5µm. Alumina coatings of 200µm thickness were prepared using the two fuel gases. These coatings were compared with alumina coatings of similar thickness prepared by plasma spraying.

Coating characteristics

Shown in Table 3 are the measured characteristics of the prepared HVOF coatings and of an air plasma sprayed coating. The HVOF alumina coating sprayed using hydrogen fuel gas had a low surface roughness measured at 1.4µm Ra. The same powder sprayed using acetylene gave a coating with a much rougher surface, measured at 4.4µm Ra. Similar to a plasma sprayed alumina coating, which had a measured surface roughness of 4.3µm Ra.

Table 3 Coating characteristics compared

The HVOF alumina coating had a higher measured Vickers microhardness when prepared using hydrogen rather than acetylene as the fuel gas. The measured values were 1215 and 879 kg/mm ² respectively.

Optical micrographs at x500 magnification of coating cross-sections for the HVOF alumina coatings are shown in Fig. 1 and 2. The HVOF alumina coating prepared using hydrogen fuel shows less evidence of apparent porosity than the coating prepared using acetylene fuel. A summary of the crystalline phases detected in each coating can be found in Table 3. The use of hydrogen fuel gave a coating consisting mostly of γ-alumina, with α-alumina present as a minor phase.

The α-alumina starting powder has been converted to a primarily γ-alumina coating. The residual α-alumina content is about 10 wt%. The HVOF alumina coating prepared using acetylene as the fuel gas showed a higher α-alumina content of about 46%, with less γ-alumina being formed during the spraying process.

The air plasma alumina coating consisted primarily of γ-alumina, with α-alumina present at about 7%.

Coating adhesion was measured using the tensile pull method as described by the ASTM standard C633-79. The HVOF alumina coatings prepared using both fuels had bond strengths of over 44 N/mm ² , similar to results for air plasma alumina.

Wear performance

Abrasive wear was measured using the dry sand rubber wheel test in accordance with the ASTM standard G65-91. A lower weight loss means better resistance to wear, see Fig.3. The results show that the HVOF alumina coating prepared using hydrogen fuel had the lowest wear rate. Use of acetylene fuel gave a coating with a much higher wear rate. An air plasma sprayed alumina coating had a measured wear rate which is considerably higher than that for the HVOF coating sprayed using hydrogen, and similar to the value obtained when using acetylene fuel.

Erosion wear was measured using a grit blast test which involved directing an air jet containing angular quartz grit on to the coating surface. The erosion test was performed on the HVOF alumina coating and a low pressure plasma sprayed alumina coating. Erosion rates were measured for an impact angle of 90°. The results are shown in Fig.4. The HVOF alumina coating prepared using hydrogen as the fuel gas again showed the lowest wear rate. It had a significantly lower wear rate than a low pressure plasma sprayed alumina coating. A much higher wear rate was observed for the HVOF alumina coating sprayed using acetylene.

The plasma sprayed alumina coatings examined in this work act as a reference with which the wear performance of the HVOF coatings can be compared. Wear resistance of the HVOF alumina coating sprayed using hydrogen fuel is significantly better than for the plasma sprayed alumina coatings.

The better wear performance of the HVOF alumina coating is an unexpected and surprising result. The present general consensus is that the HVOF process is not suitable for deposition of high quality alumina coatings, and that plasma spraying and detonation flame spraying are the preferred choice.

This work has clearly demonstrated that HVOF thermal spraying must be considered as a competing process for preparation of high quality alumina coatings.

Conclusions

The HVOF process can deposit high quality coatings of alumina when hydrogen is used as the fuel gas. These coatings demonstrate better abrasive wear performance than an air plasma sprayed alumina coating. The alumina coating also demonstrates better erosion wear performance than a low pressure plasma sprayed alumina coating. Use of an acetylene fuel gas with the HVOF process gives an alumina coating of poor quality. It cannot match the wear performance of an alumina coating prepared using hydrogen as the fuel gas.