What Lab Testing is Typically Used for Thermal Spray Coatings?
The basic purpose for scientific experimentation and lab testing is to prove or disprove a hypothesis. Lab testing in the thermal spray world uses industry standard testing procedures to prove the quality of a coating. The results can be used to proof the spray parameters, process, and set up of a coating application. When investing in a coating for a long-term, high volume production run of parts, this is typically done in the research and development phase. There are three basic types of lab testing that are beneficial for thermal spray coatings: bond strength, hardness testing, and microstructure analysis.
Bond Strength Testing
Bond strength testing examines the adhesion of the coating to the substrate surface. This is done by spraying a metal plug with the desired coating and testing according to ASTM C633. This test uses high strength epoxy to attach a coated plug to a test plug. A tensile machine then pulls the plug apart and measures at what load the coating fails. Most of the time with HVOF or HVAF coatings, the coating bond will be stronger than the epoxy resin bond. This is why the bond strength is typically listed as 10,000 psi.
Most thermal spray material and process combinations have a known range of bond strengths that are possible. Poor bond strength results usually indicate an error or issue in the application process. While bond strength can be an issue to determine coating failure versus function, hardness testing will be an indicator of the wear resistance potential of the coating.
Coating Hardness Testing
Hardness testing is only one indicator of the potential wear resistance of a coating, but it can be powerful in determining if a coating is applied within specification. Like bond strength, most material and spray process combinations have known hardness ranges. Hardness can be tested using macro or micro-hardness depending on what loads the coating will undergo in the intended operating environment.
A common hardness test used in thermal spray applications is Vickers microhardness testing using Vickers hardness testing. This test uses lighter loads and can be done on a representative sample rather than on the part itself. These are usually small coupons sprayed under the same parameters and processes as the parts. Often hardness testing is done in conjunction with microstructure analysis to get the fullest view of the coating quality.
Thermal Spray Microstructure Analysis
Microstructure analysis uses a high-powered microscope on a cross-section of a coating to examine porosity and coating structures. A test coupon is sprayed with the coating in question and the sample is cut and mounted in a mold for examination. The photos taken by the microscope are then analyzed to determine porosity and check for: cracking, delamination, oxides, unmelted particles, and/or embedded grit leftover from blasting. All of these could weaken the coating indicating an issue within the coating process.
When to Test, When Not to Test, that is the Question
While you do not need these tests on every coating or part, it is good to know that they exist and how they correlate to coating performance. Testing is valuable when using a coating material or process for the first time. It is especially important if you are planning to use a coating for a long-term, high quantity production run of parts.
These lab tests are often required for industries with an increased safety risk should coating failure occur, such as nuclear power generation or aerospace. If you are wondering if you should do any lab testing for your next thermal spray coating, it’s best to reach out to your coating provider. They may already have recent lab data they can share, or they can advise if testing would be necessary or beneficial.