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Substrate and Coating Erosion_4

Tham khảo tài liệu 'substrate and coating erosion_4', kỹ thuật - công nghệ, điện - điện tử phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 194 5 Substrate and Coating Erosion Fig. 5.20 Elastic contact time calculated for a typical substrate-coating arrangement see Fig. 5.14 for elastic constants measured strain rates between âp 1 X 104 and 4 X 104 per second for impact velocities up to rp 130 m s. 5.4.2 Material Response to Particle Impingement Depending on the contact situation materials respond either elastic or plastic to solid particle impingement. Examples are shown in Fig. 5.21. The critical particle velocity for plastic flow during particle impact is Johnson 1985 2 26 ơf Ym 4 ơf PL Pp 5.31 The threshold particle velocity for Hertzian crack formation can be derived from 5.24 in combination with Auerbach s law PH Aa dp . This procedure delivers Q 1 3 c -1 2 A -5 6 vh a5 6 T k n PP 5.32 H a 4 V 3 V 2 J Here Aa is the Auerbach constant. For rH rPL and ơf HM 5.31 and 5.32 deliver the following condition for elastic-plastic transition 5.4 Material Loading Due to Solid Particle Impingement 195 a b Fig. 5.21 Types of response to solid particle impingement Aquano and Fontani 2001 . a Elastic response with cone crack formation b Plastic-elastic response at different particle impact velocities 25 the lower drawing is adapted from Winter and Hutchings 1974 . 196 5 Substrate and Coating Erosion Fig. 5.22 Impact transition criterion for coating materials according to 5.33 dpL 3 a hm 5.33 This relationship is illustrated in Fig. 5.22. This graph reveals a size and a shape effect for erosion processes. The transition from elastic to plastic response depends on the diameter of the abrasive particles the smaller the impinging particle the higher is the probability of plastic response. Also the higher the curvature of the particle tip broken irregularly shaped abrasive particles have high curvature values the higher is the probability of plastic response. 5.4.3 Formation of Radial and Lateral Cracks A crack system as shown in Fig. 5.23 forms under certain contact conditions in brittle coatings. Radial cracks form