Tensile Test Specimens with a Circumferential Precrack for Evaluation of Interfacial Toughness of Thermal-Sprayed Coatings

Abstract

A new testing procedure to evaluate the interfacial toughness of thermal-sprayed coatings has been developed. The newly designed test specimen is a modification of the pin test with an artificially introduced weak interface, which is expected to open up easily under tensile loading and act as a circumferential precrack along the interface between a coating and the substrate. This configuration makes it possible to calculate the stress intensity factor K Int at the tip of the precrack, which can be expressed aswhere r 0 is the apparent average stress, a the crack length, R the specimen radius, and F I the geometrical correction function. Finite-element analysis was carried out to calculate the correction function F I for various values of a/R. In the experiments, the flat surface of a pin was gritblasted and a ring-shaped area from the periphery was covered with carbon using a pencil and set into a mating dice. SUS316L stainless steel was plasma-sprayed onto the flat surface of the pin and the dice. Then, tensile load was applied to the pin to break the weak interface containing the carbon and finally the unmodified coating-substrate interface. The load required to pull out the pin was measured for various specimen parameters such as a and R. The results indicate that the adhesion of the tested coatings can be represented by interface toughness of 1.9 ± 0.1 MPa m 1/2 . As a consequence, this testing procedure can be considered as a viable method to evaluate adhesion of a thermal-sprayed coating on a substrate.

A correlation between fracture toughness and cohesion strength of molybdenum thermal sprayed coatings

“…However, similar associations between results from the tensile adhesive tests and interfacial indentation tests were made for thermal sprayed coatings and it was concluded that the interfacial fracture toughness is a suitable parameter to characterize the adhesion of thermal sprayed coatings [5] [14]. To correlate the local fracture toughness with the cohesion strength of Mo coatings investigated, it was considered an average fracture toughness of the values obtained for each thermal spray method (0.419 Mpa•√m for EA, 0.623 Mpa•√m for APS and 1.081 Mpa•√m for HVOF).…”

Review of Functionally Graded Thermal Sprayed Coatings

“…Many tests of adhesion were developed basing on fracture mechanics [195]. Other methods of adhesion characterization in macroscopic and microscopic scale include the tests such as: tubular coating tensile, peel adhesion, pin, laser-shock, interfacial indentation, and scratch one [196][197][198][199][200][201][202][203][204][205]. The cohesive strength of a coating can be determined by tests of tensile adhesion, pin, shear load, and scratch one [206].…”

Rupture of alumina coatings on C35 steel: A numerical simulation

“…The main methods are the pin test, 10 the interfacial indentation test, 11 the shear adhesion test, 12 the peel adhesion test, 13 the bending tests, and the double cantilever beam test. 14 But the most commonly used is the standardized tensile adhesion test ASTM C633-13.…”

“…In a similar approach, Kishi et al. 10 adapted the pin test and Watanabe et al. 18 adopted the same ring shape carbon layer than Kishi et al.…”