ANTIFRICTIONAL MATERIALS BASED ON TUNGSTEN CARBIDE, NICKEL AND CARBON

Tungsten-nickel composite materials such as VN8 Gr2, VN20 Gr6, etc., containing up to 6% of the mass. carbon produced according to TU 88 of Ukraine 90.1067–89 have found wide application in the production of elements of mechanical seals and friction units operating under difficult conditions of dynamic loading, high contact pressures and speeds.

An important factor for ensuring favorable conditions for the long-term operation of highly loaded friction units is the presence in the composition and structure of such materials as carbon, which acts as a solid lubricant and provides a low ratio of friction. The main difficulty of the technology for obtaining composite materials of this composition is to achieve a uniform distribution of carbon throughout the sample volume, which ultimately affects the physic-mechanical characteristics of the resulting products.

At present, a technology has been developed for the production of similar materials VN8 Gr2, VN20 Gr6, etc. with improved physical, mechanical and service properties, which makes it possible to organize test and small-scale production of powder mixtures called VNS8–2, VNS20–6, etc., for manufacturing of antifriction products.

Table 1 shows the chemical composition of tungsten-nickel composite materials VN20 Gr2, VN20 Gr4 and composite powder materials obtained by the new technology, and Table 2 shows some of their physical and mechanical characteristics. In the structure of the sintered tungsten-nickel composite material VNS20-2, carbon particles have a size of less than 0.5 mm that are distributed quite evenly and form conglomerates of significantly smaller sizes, which are also uniformly distributed throughout the sample volume. This, in our opinion, leads to higher values ​​of ultimate strength in bending and hardness HRA, the values ​​of which are given in Table 2.

When choosing antifriction materials for designing friction units, the value of the static friction of two bodies at their micro-displacements relative to each other is of considerable interest. It is known that external conditions (load, speed, roughness, temperature) affect the amount of static friction no less than the nature of rubbing bodies, changing it several times.

The determination of the incomplete frictional force (static friction) of new antifriction powder hard alloys based on tungsten carbide, nickel and carbon was carried out with a change in external conditions.

The studies were carried out on the UITP installation, the kinematic diagram of which is shown in Fig. 1.

The magnitude of the loading of the tested friction pair was recorded using exemplary dynamometer DOSM3-02, and the shear force was measured by the electronic unit of the dynamometer ADSD/1 R.

The calculation of friction ratio at rest was determined by the following formula:

K o = P 1 / P 2,

where P 1 is the resistance force during the relative movement of one body over the surface of another under the action of an external force (shear force), kg;

Р 2 - normal force pressing bodies against each other (load force), kg.

Table 3 shows the values of the static friction coefficient of friction pairs depending on the magnitude of the load, the holding time under the load and the surface roughness. It is shown that the coefficient of static friction is practically independent of the holding time under load in the time interval from 5 to 60 minutes.

Table 4 shows the values of the static friction ratio of friction pairs, depending on the magnitude of the load, and the surface roughness at a holding time under load of 15 minutes.

In the course of the studies carried out, the dependences of the static friction coefficient on the value of the load and the holding time under the load for various pairs of friction were calculated.

The data obtained allow us to conclude that tungsten-nickel composite materials such as VNS8-2, VNS20-6, etc., can be recommended as antifriction materials in the manufacture of end seals and friction units instead of composite materials VN8 Gr2, VN20 Gr6, etc., produced according to TU 88 of Ukraine 90.1067–89.

At present, Technical Conditions "Antifriction products made of powder hard alloys" have been developed and agreed upon with the interested enterprises, which apply to the details of bearing assemblies from the proposed powder composite materials manufactured using the new technology.

Table 3. Dependence of K0 on the magnitude of the load and the holding time under load

No. in order	Friction pair	S = 26 kg/cm3		S = 52 kg/cm3		S = 78 kg/cm3		R, micron
		f. min	KO * 10-2	f, min	К0 * 10-2	f, min	KO * 10-2
1	2	3	4	5	6	7	8	9
1	VN20Gr2 VN20Gr 4	5	2.3	5	1,6	5	1.9	4.2-5 (5kl) 14-18 (Зkl)
		15	2.0	15	1,8	15	1.9
		30	2.1	30	1,8	30	1.8
		60	2.1	60	1.8	60	1.8
2	VNS20-2 VNS 20-4	5	1,2	5	1,2	5	1.3	0.51-0,78 <7-8kl) 0.46-032 (8kl)
		15	1,2	15	1.1	15	1.3
		30	1,3	30	1.2	30	1.3
		60	1.3	60	1.1	60	1.3
3	VNS20-2 VNS20-2	5	1.2	5	1.4	5	1.3	0,51-0.78 (7-8kl)
		15	1.2	15	1.4	15	1.4
		30	1.1	30	1.0	30	1.2
		60	1.1	60	1.3	60	1.2
4	VNS20-4 VNS20-4	5	1.6	5	1.5	5	1.3	0.46-0,52 (8kl)
		15	1.5	15	1.4	15	1.3
		30	1,5	30	1.3	30	1.3
		60	1,5	60	1.3	60	1.3
5	VNS20-6 VNS20-6	5	1.9	5	2.0	5	1.6	0.66-0,78 (7-8kl)
		15	1.8	15	1,9	15	1.7
		30	1,8	30	2.3	30	1.8
		60	2.0	60	2.3	60	1.8
6	VNS8-2 VNS8-2	5	1.9	5	1.7	5	1.5	032-0.78 (7-8kl)
		15	1.9	15	1.6	15	1.6
		30	1,8	30	1.7	30	1.6
		60	1.9	60	1.6	60	1.6
7	VNS8-4 VNS8-4	5	1.6	5	1.6	5	1.4	038-0.66 (7-8kl)
		15	1.6	15	1.6	15	1.5
		30	1,9	30	1.6	30	1.7
		60	2.0	60	1.5	60	1.7

 

Table No. 1. Chemical composition of materials
# in order	Material designation							Composition, in vol.%, ± 3%
								Tungsten carbide					Nickel				Carbon
1	VN 20Gr2							62					28				10
2	VN 20Gr4							54					26				20
3	VNS 20-2							62					28				10
4	VNS 20-4							54					26				20
5	VNS 20-6							49					24				27
6	VNS 8-2							76					12				12
7	VNS 8-4							67					11				22
Table 2. Physical and mechanical properties of materials
#			Material designation				Rockwell hardness (NCA)			Ultimate bending strength (bend.), MPa (kg / mm2), not less than				Density (U), g/cm3		Thermal expansion coefficient (a * 10-6), deg-1
1			VN20 Gr2				70,0			1078(110)				11,7-12,2		-
2			VN20 Gr4				50,0			539(55)				10,3-10,9		-
3			VNS 20-2				79,0-79,5			1176(120)				11,5-12,8		5,65-6,88
4			VNS 20-4				71,0-75,0			1078(110)				11,2-11,3		5,84-6,83
5			VNS 20-6				56,5-57,0			588(60)				9,03-9,6		5,85-6,92
6			VNS 8-2				85,0-87,0			980(100)				13,2-13,4		4,82-5,81
7			VNS 8-4				68,0-72,0			490(50)				9,6-10,3		6,45-8,83
Table 4. Dependence of K0 on the value of the load and surface roughness
#		Friction couple				S = 26 kg/cm3			S = 52 kg/cm3			S = 78 kg/cm3			R, micron
						К0 * 10-2			К0 * 10-2			К0 * 10-2
1		VN20Gr2 VN20Gr 4				1,64			1.6			1.25			0,87-0,89(7 kl ) 0.82-0,93(7 kl )
2		VNS20-2 VNS 20-4				1.3			1,21			1.15			0.13-0,14(10kl) 0,17-0.2(9kl)
3		VNS20-2 VNS20-2				1.74			1.75			1.65			0,13-0.14(10 kl )
4		VNS20-4 VNS20-4				1.82			1,88			1.92			0.17-0.2 (9kl)
5		VNS20-6 VNS20-6				1.93			2.0			1.78			0,27-0.34(8-9kl)
6		VNS8-2 VNS8-2				1,70			1.69			1.65			0,13-0,14(10kl)
7		VNS8-4 VNS8-4				2.36			2.22			2.02			0,26-0,34(8-9kl)
8		VNS20-4 VNS8-2				1.86			1.79			1.78			0.17-0,20(9kl) 0,13-0,14(10kl)