Plasma sprayed ceramic coatings for electrical purposes-necessity of process control

Abstract

Ceramic coatings with high electrical insulation capabilities can be sprayed on to metallic substrates by plasma spraying for insulation purposes, e.g. for corona rolls. However, the electrical and mechanical properties of such coatings severely depend on the spray conditions used during coating. This paper discusses the effects of spray conditions on coating properties and what parameters are essentially to be monitored for achieving high quality coatings. The paper does not confine to the classical machine parameter control techniques, however, it introduces recently developed diagnostic tools for on-line monitoring the spray particle jet and substrate conditions. I INTRODUCTIONThe low value for electrical conductivity and high dielectric strength of oxide ceramic materials such A1,0,, ZrO,, SiO, and TiO, make them popular for applications requiring electrical insulation. Beside the use of these materials for monolithic ceramics the high process temperatures and flexibility of Atmospheric Plasma Spraying allows deposition of these oxides to form a coating on all kind of substrate materials. Coatings, even up to several mm thick can be produced. The formation of A1,0, from it's molten phase shows many modifications. The high cooling rates involved cause formation of metastable phases q, y, 6 and 8, since nucleus activation energy for these phases is comparably low. These phases, Table 1 : Properties of oxide ceramic materials ceramic a-Al,O, TiO, SiO,however, show explicitly inferior electrical properties. The desired a-phase can only be formed for low cooling rates or subsequent heat treatment. A high a-content can be achieved by slow solidification which can be controlled by spraying on substrates with high temperatures and by using bigger particles leading to decreased cooling rates. Of special interest for electrical shielding of components up to very high voltages are thick coatings (1-5 mm). To manipulate phase composition and optimise residual stresses, especially in case of thick coatings, tools to monitor the process are required [1,2,3,4]. An on-line process control approach will be discussed in the following. I1 MONOLYTHIC AND COATED OXIDE CERAMICS FOR ELECTRICAL INSULATION PURPOSESThe low electrical conductivity and high dielectric strength of oxide ceramic materials such as A120,, ZrO,, Cr,O, and TiO, make them popular for insulation purposes (Table 1). For monolithic ceramics the dielectric strength is ca. 20 kV/mm and for plasma sprayed coatings depending on the thickness, the value is only a factor 0,3-0,s. Responsible for the insulation capacity of the plasma sprayed coating is the purity of the spray powder (contaminations are FeO, Si02, ...) and porosity as well as crack density and orientation. The investigations focus on development of thin(ner) coatings, to avoid residual stresses, but with improved dielectric strength.The content of a-and y-phase in the coating is determined by the melting degree of the powder which consequently determines insulative capacity. The high cooling rates involv...

A novel integrated temperature investigation approach of sprayed coatings during APS process

“…In the past, Lugscheider et al (1998a) have monitored the temperature evolution of a button-type sample with a Keller pyrometer and a thermocouple, and accordingly adjusted the cooling capacity to achieve a constant substrate temperature of 375 • C. Correlations between the mechanical properties and the temperature history of coating during high velocity oxyfuel (HVOF) process were investigated with the help of the in situ temperature control based on the thermal camera (Jokinen et al, 1999). The spray and deposit control (SDC) system has been applied to detect the substrate/coating temperature time evolutions by IR pyrometry and simultaneously the beam deflection caused by stresses generated within the coating and substrate during spraying.…”