Table 1
Critical lateral forces and values of μ determined from nanoscratch trace depths within GaN films on c- and a-axis sapphire substrates
Sample	Normal load (µN)	Coefficient of friction	Lateral force (µN)
GaN C-plane	2,000	0.105	−91.3
GaN C-plane	4,000	0.105	−200.2
GaN A-plane	2,000	0.096	−100.6
GaN A-plane	4,000	0.188	−256.2

Lin et al. Nanoscale Research Letters 2010 5:1812   doi:10.1186/1556-276X-5-1812