Influence of spraying parameters on the properties of HVOF alumina coatings

Paper presented at International Thermal Spray Conference 2-4 May 2005, Basel, Switzerland.

The HVOF spraying process has recently been considered for the deposition of dense alumina coatings for dielectric coatings in semiconductor applications and as a turbine blade tip coating in aeroengines. However, due to the lower flame temperature of the HVOF process compared to the plasma spray process it is necessary to have good control over the key process parameters to achieve the correct coating characteristics. The work reported presents the results of a design of experiment study carried out on a TopGun HVOF system used to prepare coatings of alumina. The influence of several key parameters on coating characteristics such as porosity, alumina phase type, microhardness, surface roughness and adhesion have been determined. The parameters varied included oxygen and hydrogen fuel gas flow rates, and spray distance. Based on the results of these investigations recommendations are made on the control of key parameters and the range of coating characteristics that can be expected.

1. Introduction

A number of single and mixed oxide ceramic compounds, including alumina, titania, alumina-titania, chromia and zirconia, can be deposited by thermal spraying processes.^[1] Thermal sprayed alumina offers good wear resistance and a high hardness which is retained at elevated temperatures. Alumina is used 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. Other applications include turbine blade tip coatings in aeroengine applications and as sliding and abrasive wear resistant coatings for pump and machinery components within the petrochemical, pulp and paper and textile industries.

Plasma spraying is commonly used to deposit oxide ceramic coatings. However, HVOF spraying is also used for some oxide ceramic coatings to achieve improvements in wear properties.^[2-3] In this study a design of experiment approach has been used to investigate the spraying parameter settings employed to deposit alumina coatings using theTopGun HVOF system. Due to the lower flame temperature of the HVOF process compared to the plasma spraying process it is necessary to have good control over the process parameters to achieve the correct coating characteristics.

The influence of oxygen and hydrogen fuel gas flow rates, and spray distance on coating characteristics, such as porosity, alumina phase type, microhardness, surface roughness and adhesion to a steel substrate have been determined. Based on the results of these investigations recommendations are made on the control of key parameters and the range of coating characteristics that can be expected.

2. Coating preparation

Alumina coatings were prepared on carbon manganese steel (S355 J2G3) using the TopGun HVOF system. Coatings were prepared with AI-1110-HP powder obtained from Praxair Surface Technolgies Inc. with a nominal size of 5-22µm. The surface of the substrates were prepared by grit blasting with alumina grit of mesh size 60 and then air washed and degreased immediately prior to coating.

The characterisation and adhesion test pieces were mounted on the perimeter of a turntable. The gun axis was aligned perpendicular to and directed at the turntable axis of rotation which was set to the vertical. The rotational speed of the turntable was set such that the traverse speed of the test pieces was 1ms^-1. The gun was moved up and down in the vertical direction at 2.5mms^-1 until 32 passes had been completed to deposit a coating nominally 300µm thick.

Table 1. Spraying parameters

3. Experimental procedure

A two level factorial experimental design with two additional centre points was used to investigate the role of three parameters. The three parameters examined were spray distance, hydrogen flow rate and oxygen flow rate. The design of experiment trials involved the preparation and characterisation of 10 coatings. The spraying conditions given in Table 1. The lower setting for spray distance (A-) was 152mm and the upper setting (A+) was 229mm. Likewise for hydrogen flow B- was 1530scfh and B+ was 1766scfh, and for oxygen flow C- was 540scfh and C+ 620scfh. The coatings were then examined to determine the microstructure, deposition characteristcs, surface roughness, microhardness and coating adhesion. These measurements provided the response data for the design of experiment method.

Cross sections of the sprayed coatings were prepared using standard metallographic techniques. The coating thickness was then measured and scanning electron microscopy images taken. The Vickers microhardness was measured on the coating sections using a Duramin hardness tester manufactured by Struers and a load of 300g. 10 indents were made, and the hardness value expressed as a mean value with standard deviation.

The surface roughness of the deposited coatings was measured using a stylus profilometry technique with a Surfcom 300B equipment having a 5µm diamond stylus tip and a 4mN force. Values are quoted as a roughness average (Ra) in µm.

Coating adhesion was measured according to the ASTM standard C633-01. It consists of coating one face of a loading fixture and bonding this coating to the face of an uncoated loading fixture with a high strength structural adhesive (trade name FM1000). The assembly was placed in a tensile loading machine with self aligning devices. The tensile load was increased at 1mm/min and the load at failure recorded. For each coating type, the adhesion of five test pieces was measured and expressed as a mean value with standard deviation.

These results indicate that when preparing alumina coatings by HVOF thermal spraying it is important to maintain good control of the spray distance and oxygen flow rate in particular. High adhesion and high hardness coatings arebest prepared using the shorter spraying distance of 152mm. These properties are strongly influenced by variation in spray distance but are fairly insensitive to variation (for the settings considered in this work) in the setting forhydrogen flow rate and oxygen flow rate. Using the information in Table 3, a change in spray distance of only 39mm from 152mm to 191mm will cause an estimated drop in hardness of about 10% and a drop in adhesion of about35%.

The shorter spray distance was found to cause a drop off in deposition efficiency. Table 3 indicates that deposition efficiency is also influenced by oxygen flow rate, with the higher oxygen flow rate giving an increase indeposition efficiency. By using a higher oxygen flow rate with the shorter spray distance a good combination of high hardness, high coating adhesion and good deposition efficiency should be possible. The results also show that hydrogenflow rate has quite a strong influence on surface roughness, with the higher flow rate giving the least rough surface. Based on these considerations the recommended settings for spraying an alumina coating are:

These settings are expected to give a coating having a deposit efficency of 53%, hardness = 1131 HV, α-Al2

5. Conclusions

The hardness and adhesion values of the alumina coating are strongly influenced by the distance between the spray gun and substrate surface, with the shortest distance considered in this work (150mm) giving the highest values. Arecommended set of conditions for the hydrogen and oxygen flow rates, and spraying distance have been proposed based on a design of experiment approach.

6. Acknowledgements

The authors would like to acknowledge TWI staff who contributed to this work and the project partners in the European CRAFT project: CRAF - 1999-70297, funded by the European Community under theCompetitive and Sustainable GrowthProgramme (1998-2000).

Table 3. Summary of ANOVA results

7. References

1. Pawlowski L. The science and engineering of thermal spray coatings. John Wiley & Sons, 1995.
   
   
2. Sturgeon A.J., Blunt F.J. and Harvey M.D.F. High velocity oxyfuel sprayed ceramic coatings. Fourth Euro-Ceramics, Proceedings, European Ceramic Society Conference, Riccione, Vol.9; 2-6 Oct. 1995, pp451-459, 1995.
   
   
3. Sturgeon A.J., Harvey M.D.F; Blunt F.J. and Dunkerton S.B. The influence of fuel gas on the microstructure and wear performance of alumina coatings produced by the high velocity oxyfuel (HVOF) thermal spray process. Thermal Spraying - Current Status and Future Trends, Proceedings, 14th International Thermal Spray Conference, Kobe, Japan, Vol.2; 22-26 May 1995, pp669-673, 1995.
4. Cullity B.D. Elements of X-Ray Diffraction. Addison-Wesley (1978).