HVOF coatings for prevention of gas turbine blade tip wear

The prevention of wear at high temperature of gas turbine blade tips is a major concern for gas turbine users. Current best practice for prevention of blade tip wear includes weld overlay with cobalt alloy and high velocity oxyfuel (HVOF) deposition of CoNiCrAlY coatings, but the wear, corrosion and erosion resistant properties of these alloys deteriorate at temperatures in excess of 1100°C (2000°F). However, with the increase in gas turbine combustion temperatures there is a need for coatings that can withstand service hot gas path temperatures up to 1500°C (2700°F).

A promising solution is the coating of blade tips with ceramic coatings such as alumina and yttria stabilised zirconia (YSZ) by HVOF spraying. In contrast to the porous ceramic coatings, deposited by plasma spraying and the columnar, vertically cracked YSZ coatings deposited by electron beam physical vapour deposition (EB-PVD), the HVOF process deposits very dense coatings characterised by high inter-particulate cohesion, superior adhesion to the substrate, and higher hardness.

TopGun HVOF system - used for spraying ceramic coatings

TWI's expertise

The deposition of ceramic coatings by HVOF spraying has been considered technically challenging because of their very high melting points. Nevertheless, work at TWI has demonstrated the feasibility of depositing ceramics (including alumina, chromium oxide and yttria stabilised zirconia) with the TopGun HVOF system.

TWI offers:

Process development

• Evaluation of a selection of HVOF systems
• Evaluation of spray powder consumables

Characterisation of YSZ coatings:

• X-ray analysis of phases
• Density measurements
• Surface roughness

Measurement of properties:

• Tensile bond strength
• Room temperature abrasive tests
• High temperature abrasive wear resistance against relevant wear couple
• Robustness to thermal cycling:

Full system specification

For further information, contact: power@twi.co.uk

Copyright © 2001, TWI Ltd