(b) nm 50 50 0 0.5 2.0 0.5 1.0 15 1.0 1.0 1.5 0.5 1.5 0.5 0 3000 1500 60 Contact depth, nm Contact depth, nm Fig 4. (aHc)Representative AFM images: A representing a bn particle and B representing a Si3 N4 particle (d) Nanoindentation load-displacement curves (e) Elastic moduli and ardnesses as a function of indentation contact depth of the Bn and siN4 particles and in BN interfacial layers of specimen BS-5 tests were performed using a Troboscope nanomechanical test- tion using Oliver and Pharr method [38. The hardness is given ing system(Hysitron Inc, Minneapolis, Minnesota, USA)in con- by junction with the Veeco AFM system. The Hysitron nanoindente monitored and recorded the load and displacement of the in- H denter, a diamond Berkovich three-sided pyramid, with a force resolution of about 1 nN and displacement resolution of about where Pmax is the peak indentation load, Ac is the real contact area. 0.1 nm. A typical nanoindentation experiment consists of four The elastic modulus was calculated using the following equations subsequent steps: approaching the surface; loading to peak load holding the indenter at peak load for 5 s: finally unloading E completely. The hold step was included to avoid the influence of creep on the unloading characteristics since the unloading curve vas used to obtain the elastic modulus of a material under test [36, 37]. The indentation impressions were then imaged with Ei Es the same indenter tip. The hardness and elastic modulus were where Er is the reduced modulus, S is the contact stiffness determined determined from the load-penetration curve of the indenta- from the initial part of the unloading curve, Ei, Es v and vs are the 250 Fig 5 SEM images of three fractured Si N4/BN composite specimens: (a)BS-50, (b)BS-10, and (c)BS-5tests were performed using a Troboscope nanomechanical test￾ing system (Hysitron Inc., Minneapolis, Minnesota, USA) in con￾junction with the Veeco AFM system. The Hysitron nanoindenter monitored and recorded the load and displacement of the in￾denter, a diamond Berkovich three-sided pyramid, with a force resolution of about 1 nN and displacement resolution of about 0.1 nm. A typical nanoindentation experiment consists of four subsequent steps: approaching the surface; loading to peak load; holding the indenter at peak load for 5 s; finally unloading completely. The hold step was included to avoid the influence of creep on the unloading characteristics since the unloading curve was used to obtain the elastic modulus of a material under test [36,37]. The indentation impressions were then imaged with the same indenter tip. The hardness and elastic modulus were determined from the load–penetration curve of the indenta￾tion using Oliver and Pharr method [38]. The hardness is given by H ¼ Pmax Ac ð1Þ where Pmax is the peak indentation load, Ac is the real contact area. The elastic modulus was calculated using the following equations: Er ¼ ffiffiffi p p 2 d S ffiffiffiffiffi Ac p ð2Þ 1 Er ¼ 1 m2 i Ei þ 1 m2 s Es ð3Þ where Er is the reduced modulus, S is the contact stiffness determined from the initial part of the unloading curve, Ei, Es, νi and νs are the Fig. 4. (a)-(c) Representative AFM images; A representing a BN particle and B representing a Si3N4 particle. (d) Nanoindentation load–displacement curves. (e) Elastic moduli and hardnesses as a function of indentation contact depth of the BN and Si3N4 particles and in BN interfacial layers of specimen BS-5. Fig. 5. SEM images of three fractured Si3N4/BN composite specimens: (a) BS-50, (b) BS-10, and (c) BS-5. X. Li et al. / Materials Science and Engineering C 28 (2008) 1501–1508 1503