Прунько, І., І. Богатчук, С. Криштопа, and Б. Долішній. “Optimization of Process Parameters Electrical Spark Hardening Oil and Gas Transportation Technology”. Problems of Tribology, vol. 66, no. 4, July 2014, pp. 43-48, http://tribology.khnu.km.ua/index.php/ProbTrib/article/view/267.