TWI Industrial Member Report Summary 704/2000
I J Munns and R McCarthy
Background
Increasingly, thermally sprayed coatings are being used to enhance the properties of established materials to suit specific applications. Typical examples include the use of carbide coatings to improve wear resistance, zirconia thermal barrier coatings to permit exposure to higher temperatures and aluminium or zinc based coatings to improve corrosion/oxidation resistance. The benefit provided by any coating system is dependent on its quality - if a coating becomes cracked, delaminated or damaged in any other way, its effectiveness is reduced and the corresponding benefits provided by the coating may be lost.
Currently, coating quality is ensured by monitoring and carefully controlling the process parameters during spraying. Additional checks on the spraying process may be carried out by destructively testing specially prepared test coupons. Typically, this involves adhesively bonding a threaded loading fixture to the coated coupon and then attempting to pull it off in a tensile test machine. The stress to failure is quoted as a measure of coating adhesion. Despite these checks, there is still a requirement for an NDT method capable of assessing coating quality in actual components rather than small test coupons. This report helps to address this need by providing an up-to-date review of all NDT methods suitable for the inspection of thermally sprayed coatings.
Objective
- To provide a critical review of all NDT methods relevant to the inspection of thermally sprayed coatings, thus defining the capabilities of existing techniques and highlighting lines of future development.