Methods and Formulas to Determine Internal Deposit Stress in Applied Metallic Coatings
Internal stress exists in electroplated and chemically applied metallic coatings. This paper reviews the test procedures for measuring deposit stress and the formulas used to calculate stress values. Many formulas used require modification to obtain actual internal stress values. Errors in this regard are examined and common mistakes are explained.
by Frank Leaman, Specialty Testing and Development Company, York, Pennsylvania, USA
Editor’s Note: This paper is a peer-reviewed and edited version of a presentation delivered at NASF SUR/FIN 2014 in Cleveland, Ohio on June 10, 2014. A printable PDF version is available by clicking HERE.
Internal stress exists as an inherent force within electroplated and chemically applied metallic coatings. This induced stress can be tensile or compressive in nature, causing the deposit to contract or expand in relation to the base material. High levels of stress in deposits produce micro-cracking and macro-cracking in the applied layers, and in severe cases produce a lack of deposit adhesion in the form of blistering, peeling and flaking, wave-like ripples in electroforms, and accelerated corrosion and wear failure. This paper reviews the methods and test procedures for measuring deposit stress and the formulas employed for calculating stress values. Many of the formulas used to calculate deposit stress require modification to obtain the actual internal stress value. Errors in this regard are examined and common mistakes in test methods and practice are explained.
Keywords: internal stress, metallic coatings, coating stress, stress measurement, sulfamate nickel
Internal stress exists as an inherent force within electroplated and chemically applied metallic deposits. This induced stress can be tensile or compressive in nature, causing the deposit to contract or expand in relation to the base material. High levels of stress in deposits produce micro-cracking and macro-cracking, and in severe cases produce a lack of deposit adhesion in the form of blistering, peeling and flaking. In extreme cases, wave-like ripples in electroforms, accelerated corrosion and deposit wear failure can also occur.
This paper reviews test methods, procedures and formulas used to determine the internal stress in applied metallic coatings. A comparative study for the determination of internal tensile deposit stress as plated from a semi-bright sulfamate nickel plating electrolyte was completed for the Spiral Contractometer Method and for the Deposit Stress Analyzer Bent Strip Method. Spirals were used in this study to compare the former style contractometer design and the recently designed style with and without a masked surface on the spiral inside diameter. Also, the formulas that are frequently used to calculate deposit stress values in applied metallic layers were evaluated. Limitations of these formulations, accuracy of the results and frequent errors in their use are also reported.
Two primary ways that are in use worldwide to evaluate internal deposit stress in metallic coatings are the spiral contractometer and the bent strip methods. The spiral contractometer test procedure is defined in the American Society for Testing Metals Standard B636–84. The bent strip method referred to as the Deposit Stress Analyzer method is currently in the final step of qualification to become an ASTM Standard. Each of these test procedures is applicable for determining both tensile and compressive stressed deposits. Several other test methods have been used in the past, but these have not been put into common practice. The stress meter makes use of a disk that bows inward or outward as deposition commences depending on the nature of the stressed deposit. Accuracy and stripping metal deposits from the disk remain problematic. Another method to determine deposit stress is based on measuring the change in the length of a substrate material that is caused by stress within an applied metallic coating. This method yields consistent results, but the equipment set-up is complicated and there has never been a manufacturing source for its use.
The spiral contractometer method uses a stainless steel spiral having a surface area of approximately 13 in2 (33 cm2) as the test piece. In the test procedure, the upper end of the spiral is held in a stationary position while the lower end is free to rotate as deposition of a stressed coating is applied. Induced stress that is compressive in nature will cause a spiral to contract by winding tighter, while a tensile stressed coating will cause a spiral to unwind. This movement is transferred to a dial measurement disk that is free to move at the top of the contractometer around a measurement scale that displays the spiral movement in degrees as it occurs (Fig. 1).