Fabrication of NiAl / diamond Composite Based on Layered and Gradient Structures of SHS System

In this paper layered and gradient structures of Ni-Al SHS system were adopted to manufacture Ni-Al/diamond composites. The effect of the layered and the diamond mesh gradient structures of Ni-Al/diamond on the SHS process and the microstructure of the composites were investigated. It is found that with the increasing of the number of layers, the combustion wave velocity is decreased. The combustion wave velocity for diamond mesh size gradient structure of Ni-Al SHS is faster than that for the layered structure. A well bonding can be formed between diamond and the matrix in layered and gradient structure NiAl/diamond composites due to the melt of Ni-Cr brazing alloy.


Introduction
Because of the extreme hardness, diamond is widely employed as an abrasive element in different abrasive tools [1].Hot-press sintering, electro-plating and high temperature brazing can be used to manufacture metal bond diamond tools [2][3][4].The chemical bonding is obtained between diamond grits and brazing alloy such as Ni-Cr and Cu-Sn-Tin high temperature brazed diamond tools [5,6], and the retention ability of diamond grits is increased in comparison with other methods [7].
Recently, self-propagating high temperature synthesis (SHS) has been introduced to manufacture the diamond composite materials.The SHS method allows producing multi-layer composites with TiC or MoC binder and gradient diamond concentration [8][9].Ti-B SHS was used to fabricated the multi-layered and the functional gradient diamond containing materials (FGM) [10][11][12].In our previous studies, Ni-Al SHS was also used to prepare a novel diamond grinding tool in which the compressive strength of the diamond grits was decreased about 20 % after Ni-Al SHS reaction [13].To ease the fast propagating rate and the high temperature in the self-propagating high-temperature, a mixture of Ni-Cr-P, Cu and B powder was used as a dilute for modifying the microstructure of the reacted product Ni-Al matrix grinding tool material [14][15][16].
In present study the layered and the gradient structures are introduced in Ni-Al/diamond composite in order to optimize the design of the diamond tool's structure and material.The influences of the layers' number and the gradient structure of diamond grits mesh size on the SHS process and the microstructure of reacted products are to be investigated.

Experimental
The raw materials for this experiment are shown in Tab.I.The diamond grits are from Element Six with mesh size of 60/70, 120/140, and 325/400.).The diamond concentration in each layer for mesh size gradient structure is 5 wt.%.And layer and gradient structures Ni-Al/diamond composites were coldpressed into Ф8 x 15 mm cylinder under the pressure of 60 MPa, as show in Fig. 3.The SHS samples were ignited by the tungsten with diameter of 0.28 mm, a furnace with the vacuum of 0.1 Pa.The flame front propagation velocity was measured by recording the whole combustion event with a color CCD video camera (Sony SR300E) at 25 frames/s.The video files recorded were separated into frames by the software (Ulead Video Studio 10.0) the exposure time of each recorded image was set at 0.04 s.The total time of combustion wave propagates from top to bottom is recorded, and the SHS process propagating velocity can be obtained by v = H/t, H is the height of sample and t is the combustion time.In this paper, all combustion processes begin at 0 s when the samples are ignited on the top.After SHS, the surface and longitudinal section of the sample was ground by the diamond discs (300-1200 US mesh) and diamond paste (2μm), then etched by asolution (HNO 3 :HCI:CH 3 COOH = 1:1:1 in volume fraction).Microstructural characterizations of all samples were carried out on HITACH S3400 scanning electron microscope.The phases in the different samples were identified by X-ray diffraction (XRD).X-ray energy dispersive spectroscopy (EDS) is used to map the elemental distribution between SHS matrix and diamond grits.It can be found in Fig. 4 that with the increasing of the number of layers the combustion wave velocity is decreased.More layers means that there is higher amount of dilute (Ni-Cr-Cu-B) which may absorb more reaction heat of Ni-Al SHS, and higher amount of Ni-Cr brazing alloy may be melted.Thus the propagating rate and combustion temperature of Ni-Al SHS process are eased severely.The easing effect of B, TiC and VN has also been found in some other SHS systems [17][18][19].

Tab. II
The SHS reaction processes of 3, 4, 6 and 10 layers of SHS are presented in Fig. 5, 6, 7 and 8.For 3 layer structure, the combustion wave velocity is 15 mm/s.From 0.48 s to 0.88 s, the combustion wave velocity isn't homogeneous.It can be seen that the color of the flame is faint yellow (Fig. 5).It is found that in 4 layers structures the SHS process is sTab., and the shape of sample is well kept as shown in Fig. 6.The combustion wave propagates from top to bottom in a relatively sTab.manner with the velocity of 14.42 mm/s.The combustion wave velocity is 13.89 mm/s, the combustion wave spread in Ni-Cr-Cu-B+D layer is slower than that of Ni-Cr-Cu-B+Ni-Al layer in 6 layers structure as shown in Fig. 7.The SHS process of 10 layers structure sample is shown in Fig. 8.It also can be seen that the process is sTab., the reaction time is 1.52 s, and the combustion wave velocity is 9.87 mm/s.

Microstructure of layered structure of Ni-Al/diamond
The microstructure characteristics of layered structure are show in Fig. 9.Some pores can be seen on the surface of Ni-Al layer, which is a normal microstructure in different SHS reacted bulk products.It can be also found in Fig. 9(c) in the diamond layer a well interface bond between diamond grits and SHS bond is formed, which may provide a better retention of the diamond grit.Pore

SHS process of diamond mesh size gradient structure
The SHS process of mesh size gradient structure is shown in Fig. 11.It can be found that the combustion mode is a steady SHS mode.The combustion wave spread is faster in gradient structure than in layered structure.This is because in diamond mesh size gradient structure Ni-Al SHS can spread continuously without the intermittent layer (diamond layer).The SEM images of the microstructure of each part after SHS process are shown in Fig. 12.It can be found that the diamond grits are well embedded into the Ni-Al matrix, indicating the interfacial bonding strength is strong.Fig. 13 indicates the element line scanning of carbon, Cr, Ni and Al.It should be noted that Cr element form an enrichment region on diamond surface, which is found in different studies on brazing of diamond [21][22][23].This is because that diamond has a strong tendency to react with Cr to form chromium carbide such as Cr 3 C 2 andCr 7 C 3 [24].

Conclusions
The SHS process of layered structure is steady.With the increasing of the layers' number, the combustion wave velocity is decreased.The combustion wave velocity in diamond layer is slower than that in Ni-Al layer.In the diamond mesh size gradient structure, the combustion wave spread is faster in gradient structure than in layered structure, without the effect of intermittent layer (diamond layer).In the layered and gradient structure of Ni-Al/diamond composites, well bonding between diamond and Ni-Al matrix can be formed.

Fig. 3 .
Fig. 3.The schematic illustration of SHS observation and the preparation of the grinding tool.

Fig. 4 .
Fig. 4. Effect of the number of layer on combustion velocity.

Fig. 5 .
Fig. 5. Images of SHS process of three-layer structure sample.

Fig. 6 .
Fig. 6.Images of SHS process of four-layer structure sample.

Fig. 7 .
Fig. 7. Images of SHS process of six-layer structure sample.

Fig. 8 .
Fig. 8. Images of SHS process of ten-layer structure sample.

Fig. 9 .
Fig. 9. (a) The schematic illustration of four-layer structure of SHS, (b) SEM images of reacted Ni-Cr-Cu-B+Ni-Al layer and (c) SEM images of diamond grits in Ni-Cr-Cu-B+D layer.

Fig. 11 .
Fig. 11.Images of SHS process of mesh size gradient structure.

3. 4 .
Microstructure of diamond mesh size gradient structure

Fig. 1. Schematic diagram of multilayer design of SHS system.
The parameters of layered structure.