velocities of the atoms due to lapping. An optimum size of 50a×15a×30a was obtained， consisting of 183,930 atoms. Moreover，the periodic boundary condition was used in the z-direction to reduce the effects of the simulation scale. The specimen included three kinds of atoms ，namely ：boundary atoms，thermostat atoms and Newtonian atoms.To restrict the rigid-body motion of the specimen，the boundary atoms in the left and bottom layers of the specimen that were fixed in space were used to contain the Newtonian atoms.Thermostat atoms were also used to ensure reasonable outward heat conduction away from the control volume.Thermostat atoms and the Newtonian atoms obey the Newton’s second law.The top surface of the specimen was（100）surface，which was exposed to the grit.The spherical diamond grit had a radius of 8a，consisting of 17,116 atoms.And it slid on the specimen with the depth of h.

Before carrying out the molecular dynamics simulation on the lapping of diamond，it is important to ensure that the chosen potential function gives a reliable result for the simulation. Tersoff potential was used in the present simulation to dictate the interaction among the diamond atoms in this simulation[14]. The parameters in Tersoff potential for carbon were as follows ：A=1,393.6 eV，B=347.6 eV，λ=34.879 nm.1， μ=22.119nm.1 ，β=1.572,4×10.7 ，n=0.727,51 ，c=380,49 ，d=4.384，h=.0.570 58，R=0.18 nm，and S=0.21 nm. Positions and velocities of the atoms were determined by the Verlet method as demonstrated by Maekawa and Itoh[15].To simulate lapping

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under room-temperature conditions，the diamond atoms were arranged in a perfectdiamond cubic structure with the lattice parameters equal to their equilibrium values at an ambient temperature of 293 K. The ambient

temperature was maintained by scaling the velocities of the thermostat atoms at every special time step.In this simulation，the 0.5 fs was selected as the time step to obtain a high accuracy.

This simulation was calculated by the Lammps software[16]，and visualized by the VMD software[17]. The velocity of the lapping was 100a with 1.5a in cutting depth and 40a in lapping length. Before the simulation，the specimen had been relaxed for 10 000 time steps in order to maintain the thermal balance. 1.2 Experiment

The test apparatus of lapping experiment is shown in Fig.2.The abrasive used was diamond grit with an average radius of 0.1 μm.They were coated on the scaife in a ring with a radius of 120 mm.The diamond cutting tool was fixed on the arm by a special fixture.Then，the

tool was lapped with the scaife running at 3 000r/min(ca.38 m/s)，under a load of 5 N which was obtained by adjusting the place of the weight. The debris was collected after 30 min lapping.Thereafter，the XRD studies were carried out by SHIMADZU XRD-6000.

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Fig.2 Schematic diagram of the lapping apparatus

2 Results and discussions

2.1 Molecular dynamics analysis

The 3D view and cross-section view of the simulation are shown in Fig.3. The crystal lattices near the diamond grit are distorted when the diamond grit cuts into the specimen.The region including these crystal lattices is half-ellipse in shape.The region is under the diamond grit and a bit left to the center o. And the major axis of the ellipse is in the same direction as the composition of forces. Furthermore，this region moves left as the diamond grit slides.

As shown in Fig.4 ， A1+A2

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occurs.So this state is the cutting state accompanied by ploughing.

Fig.3 Microstructure of specimen after the grit sliding

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