An HVOF extension gun for spraying internal diameters down to 5” (127 mm) has been developed and evaluated. The spray gun utilizes vortex stabilization of the flame which permits operation with a minimal length combustion chamber. Small spray distances and subsequent reduced particle dwell times present a challenge for heating and accelerating powders to achieve typical HVOF coating benchmark properties such as hardness, density and residual stresses. Initial coating characteristics and process/system capabilities are discussed.

HVOF-sprayed alumina appears to be well suited for applications in semiconductor devices. This paper investigates the influence of HVOF spraying parameters on the electrical properties of alumina layers. Diagnostic tests show that small changes in gas ratios and flow rates can significantly alter particle and splat characteristics as well as the dielectric breakdown strength of the coatings. A large number of parameters are changed in order to assess the extent to which electrical properties can be controlled. Paper includes a German-language abstract.

BaTiO3 has been successfully sprayed by HVOF to produce dense 25-150 µm thick deposits for use as dielectric and capacitive layers within prototype multilayer conformal electronics. Parameter optimization has been shown to play a critical role in the effective spraying of these materials as thin structurally homogeneous deposits. The affect of standoff distance and combustion chamber size on the phase structure of the coatings have been studied and related to the dielectric properties of the layer. The proportion of crystalline to amorphous phase was found to be critically dependent upon the degree of melting of the particles in the flame and the rate of cooling of the deposits. The crystalline/amorphous ratio is directly related to the dielectric properties of the layer with greater crystallinity giving higher values of dielectric constant. Microcracks and splat/splat interfaces are also believed to adversely affect the dielectric properties. The maximum dielectric constant (K) values achieved using the HVOF method for deposition have been in the range 70-115.

Inside diameter (ID) spraying with a two-wire gun for thermal spraying has become an established practice. While the simplicity of the two-wire arc process is tempting, there is a marked difference between the coating microstructures obtained with standard two-wire arc and two-wire arc ID systems. In conventional two-wire arc spraying, the arc is partially surrounded by the nozzle, which leads to the fused tips being enveloped by the atomizing gas flow. In comparison, the right-angled arc is unprotected if the airflow hits the arc at 90 degree. In this paper, the particle formation process is investigated using diagnostic tools such as Laser Strobe Vision (Control Vision) and DPV2000. Although the data collected may not fully encompass the breadth of the phenomena associated with two-wire arc spraying, a comparison was made between the 0 degree and 90 degree spray processes. Paper includes a German-language abstract.

WC-based coatings have found extensive application in industries requiring wear resistance. WC-12% Co coatings were applied with a variety of high speed combustion spray guns (including HVAF) with various spray parameters. This paper aims to investigate the influence of the particle velocity, the particle temperature and the spray atmosphere on the decarburisation reactions and phase changes that take place during spraying. XRD and SEM used BSE image processing to characterize and quantify these responses. A comparison between temperature and speed measurements as well as the XRD results showed that the decarburisation is influenced not only by the temperature but also by the residence time and the entrainment of oxygen. Paper includes a German-language abstract.

The erosion behavior of yttria stabilized zirconia thermal barrier coatings is investigated with respect to powder particle size. Solid particle erosion experiments were conducted at room temperature to determine the mechanism of erosion for ceramic thermal spray coatings. Testing was carried out on as-sprayed as well as thermally cycled specimens. Porosity and bend testing measurements indicate that a decrease in porosity and an increase in inter-lamellar strength leads to an increase in the erosion resistance of ceramic thermal spray coatings.