测试 and Evaluating Thermal Spray Coatings

什么是热喷涂?

Thermal spraying is a popular method of applying a metallic, 陶瓷, 塑料, 或复合涂层到器件或材料上. The process finds uses across a wide range of industries, 包括医疗, 航空航天, 制造业, 和能源. Coatings are largely used to increase certain material properties in a device or component. 

例如, a part that is going to be subjected to extreme heat may be coated with a certain formulation that will increase its thermal resistance. Thermally-sprayed, porous coatings are often used on medical implants because the porous surface encourages bone in-growth and fusion of the device. Coatings may also be applied to components in order to provide increased wear and corrosion resistance. 

The coating material starts in a powder or wire form, and is heated to a molten state before being rapidly accelerated toward the object or material in the form of micrometer-sized particles. This acceleration is typically achieved using electrical arc discharge or combustion. As the coating particles begin to accumulate, the coating is formed. 

Advantages of thermal spraying over other coating processes 

• Allows for the application of thick coatings (up to 10+ mm)
• Larger application area and higher disposition rate than other processes
• Does not cause significant heating of the material surface, resulting is less surface degradation (also allows for coating flammable surfaces)
• Great flexibility in the type of material coating applied
• Combination coating with various materials is possible
• 不需要核聚变就能成键

Mechanical 测试 of Thermal Spray Coatings

Mechanical testing on thermal spray coatings is a critical stage in the 制造业 and design process. Verifying the strength and wear resistance of coating formulations allows engineers to verify and develop processes to produce stronger and more effective coatings. 

典型的机械涂层测试包括: 

• 销盘磨损(ASTM G99) Pin-on-disk wear testing is an effective method of characterizing the coefficient of friction, 摩擦力, 以及两种材料之间的磨损率. 在这个摩擦学测试中, a stationary disk articulates against a rotating pin while under a constant applied load. This test is typically utilized as a comparison study between multiple coating formulations prior to final design and 制造业. 
• Tensile and Shear 测试 (ASTM F1147, F1160, and F1044) Mechanical testing of coatings often require specimen preparation to create a testable sample. The coating is typically set and mounted to a metallic fixture using epoxy. The resulting test sample is then tested in tension per ASTM F1147 or shear per ASTM F1044 until failure. Shear fatigue is also often performed on coating test samples in accordance with ASTM F1160. Adhesion of the coating to the fixture is critically important for these strength tests. 
• Taber磨损(ASTM D4060) Taber磨损是一种经济有效的方法, 简单的, and quick method of gathering comparable wear data for multiple coatings. The coating is subjected to cyclic abrasive force using specialized taber wheels. Pre- and post-test measurements are taken for generating mass-loss data. 
• 摩擦系数(ASTM D1894) Coefficient of friction testing consists of measuring both the static and dynamic 摩擦力s between two specified materials. Static resistance is the force that is required to initiate motion, while dynamic resistance is the 摩擦力 required to maintain motion. 
• 耐腐蚀性(ASTM B117) The corrosive resistance of a coating can be measured using a salt spray environment. ASTM B117, the coating is subjected to an extreme corrosion environment for a period of time and evaluated for signs of a corrosive presence post-test.

Metallurgical Evaluation of Thermal Spray Coatings

Metallurgical evaluations are the most commonly performed tests for thermal sprayed coatings. 

通常执行的评估包括: 

• 金相评价 A general metallographic evaluation of thermal spray coatings can generate a wide range of data, 包括:微观结构, 厚度, 粗糙度, 孔隙率或空隙含量, 氧化夹杂物, 还有未融化的颗粒, 以及裂缝的检测, 分层, 接口污染, 以及其他涂层缺陷. 
• 硬度测试 Hardness testing is also regularly performed on coating samples to obtain average hardness values as well as the hardness values through the 厚度 of the coating.  Common coating hardness test methods are Rockwell, Superficial, Vickers, and Knoop.

Sample preparation is very important for all metallurgical evaluations, because poor preparation can directly affect the appearance of the sample being evaluated. Typical sample preparation includes sectioning, cleaning, mounting, and polishing. All of these steps are performed using coating-specific procedures so as not to damage or cause any unnatural imperfections on the evaluation surface. After the mounted sample is prepared, the evaluation or test is then carried out.