Effects of laser surface modification on microstructures and properties of copper based PM friction plates

Materials

The finished products of copper based PM friction plates treated by the conventional sintering technique have been used in this study as the experimental samples. The friction plate is ring with 522 mm of outer diameter and 471 mm inner diameter. The thickness is 6 mm including 1 mm copper based PM friction layer covered on the surface and 4 mm 65Mn steel as the core plate to increasing the strength of PM layer. The schematic diagram is showed in Fig. 1.

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Figure 1

Schematic diagram of copper based PM friction plate

The chemical compositions of friction material are: 15 wt-% graphite, 3 wt-%SiO₂, 6 wt-%Zn, 3 wt-%Pb, 3 wt-%Sn, remain Cu. Its physical properties are showed in Table 1. The morphology for the cross-section of friction plate is showed in Fig. 2.

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Figure 2

Morphology for cross-section of friction plate

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Table 1

Physical properties of friction material

Density, g cm−3	Hardness, HB	Thermal conductivity, W m−1 K−1	Heat capacity, J kg−1 K−1
4·7–5·1	14–20	29	610

Experimental set-up

The general arrangement for broad beam laser modification on the copper based PM friction plate is showed in Fig. 3. The laser generator is HJ cross-flow laser. The wave length is 10·6 μm, and the beam mode is multimode. A broad beam scanning system is used to obtain a 25×2 mm spot size on the plate surface, which can avoid overlapping of single beam scanning and increase efficiency. Figure 3 also shows the principle of broad beam scanning system. First the laser beam is focused by a focusing mirror which can move up and down to adjust the focus position. Then the single laser beam is reflected through a high speed rotating polygon mirror to a broad beam. The broad beam laser is across the plate which moves following the rotating work table. The laser processing parameters used in this study are a laser power of 1·3 kW and a speed of 150 mm min⁻¹.

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Figure 3

Schematic sketch showing processing of broad beam laser modification on copper based PM friction plate

Microstructural observation

Microstructural observation was carried out for the copper based PM friction material layer using both a scanning electron microscope (SEM, AMRAY-1000B, voltage: 25 kV) and a transmission electron microscope (TEM, H-800, voltage: 200 kV). Etchant used for metallographic examination was a 5%FeCl₃ in H₂O solution for copper.

Hardness test

A ΠMT-3 microhardness tester was used to measure the microhardness of α-Cu in the copper based PM friction materials using a load of 20 g and a hold time of 10 s. The apparent hardness of friction material layer was tested using an HD-187·5 Brinell hardness tester with a load of 62·5 kg and a hold time of 10 s.

Properties test

The properties comparing tests have been carried out between the laser treated and the untreated copper based PM friction plates using an MCS200 wet clutch dynamic property tester to investigate the change of static friction coefficient, dynamic friction coefficient, wear resistance and allowable heat load value according the China National Standard GB/T15141-94 (refer to ISO 7881-1987). The 516-1 lubricant was used to cooling and lubricating at the flow rate of 6 mL cm⁻² min⁻¹ and the temperature of 50±5°C. The 65Mn steels were used to as mating plates whose surface roughness was 1·6 μm. The friction plate and mating plate contacted each other at the frequency of 2 times/min, the duration of 1 s and the specific pressure of 1 MPa. The inertia uses 4·66 kg m². The rotational speed was 1700 rev min⁻¹. The plates were used the contact of 50 and 1000 times in the dynamic friction coefficient test and wear resistance test respectively. The rotational speed of 0·304 rev s⁻¹ was used in the static friction coefficient tests. In the allowable heat load value tests, it began with the rotational speed of 1700 rev min⁻¹ and the contact of 20 times. The test would continue under the speed increased by 20% until the friction plate was failure. The failure criterion was the generating of unstable contact that the torque time curve appeared hump shaped.

Effects of laser surface modification on microstructures of friction material layer

The X-ray diffraction analysis results show that, the copper based PM friction material layers are mainly composed of α-Cu and graphite for both laser modified and unmodified. Calculated by the method of semiquantitative analysis, the relative contents of α-Cu and graphite nearly have no change too. The electron probe microanalysis results show that the elements composing the friction layer also have the same distribution.

Figure 4a and b shows the SEM micrographs of unmodified and laser modified copper based PM friction material layer respectively. The grey regions are Cu, and the black regions are graphite. The white substance is SiO₂ which is embedded in the PM layer as particles. In the processing of friction plates are sintered in the heating furnace, the copper particles are polymerising, joining and growing up to the irregularity aggregates, which form the metallic frame in the PM layer. At the same time, the Sn and Zn elements dissolve in Cu. It results in the generating of α-Cu solid solution. Cu and Pb elements are not dissolved each other, so Pb element is in the α-Cu as particles, and a small amount of it distribute in the graphite.

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Figure 4

Microstructure of copper based PM friction material layer (SEM)

Comparing Fig. 4a and b by a large region observation, we can see that the α-Cu is refining after laser modification. This is determined by laser surface modification process, and caused by the high speed heating and cooling. The α-Cu aggregates generate the edge solid phase dissolution. The large agglomerate aggregates of α-Cu break down to small strips, which increased the contact area of the copper and graphite. The α-Cu can wrap the graphite effectively, which would improve the bonding strength.

Figure 5a and b shows the TEM micrographs of the α-Cu corresponding the grey region shown in Fig. 4a and b respectively. After laser modification, the α-Cu aggregates are composed of nanocrystal which is shown in Fig. 5b. The grain size is ∼15 nm. Figure 6a and b shows the electron diffraction patterns of α-Cu corresponding to Fig. 5a and b respectively. The calibration of electron diffraction patterns shows that, the coppers are face centred cubic structure both before and after laser modification. Figure 6a shows the typical electron diffraction pattern of poly-crystal including single crystal. In the Fig. 6b, besides the typical electron diffraction pattern of poly-crystal, it includes lots of diffractive spots which irregularly distributed around the diffraction rings. It is the characteristic of electron diffraction pattern for nanocrystals structure.

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Figure 5

Images (TEM) of α-Cu

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Figure 6

Electron diffraction pattern of α-Cu

Effects of laser surface modification on density and hardness of friction material layer

Before laser surface modification, the density and the apparent hardness of the copper based PM friction material layer is 4·831 g cm⁻³ and 20·4 HB respectively. The microhardness of α-Cu is 101 HV0·02. After laser surface modification, the density is 5·123 g cm⁻³, which is increased by 6%. The apparent hardness is 23 HB, which is increased by 12·7%. The microhardness of α-Cu is 115 HV0·02, which is increased by 14%.

The PM friction material layer generates pyknosis by laser surface modification. The generation of nanocrystal enhances the microhardness of α-Cu. The improvement of the density of friction material layer and the microhardness of α-Cu cause the increase of apparent hardness of friction material layer.

Effects of laser surface modification on properties of friction plates

The wet clutch dynamic properties tests show that, after laser surface modification treatment, compared with original untreated state, the static friction coefficient of copper based PM friction plate increases from 0·136 to 0·146, which is increased by 7·4%, the kinetic friction coefficient increases from 0·065 to 0·071, which is increased by 9·2%. At the same time, the wear loss decreases from 0·009 mm down to 0·006 mm, which is decreased by 33%, the wear rate decreases from 6·49×10⁻¹⁵ to 3·27×10⁻¹⁵ m³ J⁻¹, which is decreased by 49·6%. The wear resisting property is improved obviously. And the allowable heat load value increase from 3·33 to 4·67, which is increased by 40%. The heat resistance has been improved markedly.

The wear resisting property and thermal shock resistance of friction plates are increased by laser surface modification due to the increasing of density and hardness, the refining of α-Cu and the generating of nanocrystals. As a result of the special structure and a large volume fraction interface of nanomaterials, its plasticity, impact toughness and fracture toughness have greatly improvement compared with the coarse grained materials. It is isotropic in the nanomaterial, so the pile-up of dislocation is difficult to generate at the interface. Therefore the stress concentration would reduce significantly. Thus the probability of the generating and propagating of cracks would reduce obviously.