The influence of the graphene additive on mechanical properties and wear of hot-pressed Si3N4 matrix composites

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Abstract

The study concerns silicon nitride based composites with an up to 10 wt% graphene nonoflakes (GNP) addition. An XRD and Raman analysis of hot-pressed prepared composites were conducted. The microstructural SEM observations supported by element EDS analysis were also made. Elastic and mechanical properties (Knoop hardness, bending strength, fracture toughness) were measured. The manufactured sinters were put to friction coefficient and wear tests. Wear scars on discs after the abrasion tests were observed by optical and SEM microscopy. Performance examinations of hot-pressed cutting tools were performed.

Introduction

The silicon nitride material is commonly used for producing cutting tools, bearing balls, and other parts of devices working in different conditions.1, 2, 3 Polycrystalline working ceramic machinery parts should have good mechanical properties, such as bending strength, fracture toughness or abrasion wear, even at elevated temperatures.4, 5, 6 They have a very good resistance to thermal shock and oxygen corrosion for ceramic material.7, 8 The typical commercially manufactured silicon nitride material shows thermal conductivity of about 30 W mK−1.9 For cutting tools and sealing applications, the thermal conductivity coefficient is a very important parameter, especially if the materials must work in dry conditions. In their earlier studies, the authors showed that the addition of GNP graphene to the silicon nitride matrix can significantly improve thermal conductivity in the perpendicular direction to the applied pressure during the sintering process.10 It is also possible that the graphene phase addition to the silicon nitride can significantly improve the electrical properties and allows for shaping by electro erosion machining. Single phase graphene, apart from its excellent thermal and electrical properties, also has very good mechanical strength and a low friction coefficient in the major crystallographic direction11, 12 which was a reason for a few studies concerning mechanical and tribological properties of silicon nitride/graphene composites. For preparation of such materials HIP and SPS techniques are usually used.13, 14, 15, 16, 17 In the case of the HIPed materials graphene was added in a quantity of up to 3 wt%. Fracture toughness, lower abrasive wear and lower friction coefficient increase is observed.13, 14, 15 In the case of silicon nitride/graphene composites derived from the SPS method, an increase in the fracture toughness and bending strength was also confirmed.16, 17

The presented paper is a continuation of the authors’ previous work on thermal properties of silicon nitride/graphene composites.10 The first part of this paper focuses on mechanical properties and anisotropy measurements of hot-pressed derived silicon nitride/graphene composites. Further, the influence of different amounts of the graphene phase on wear resistance and the friction coefficient of composites was determined. Performance tests of the manufactured cutting tools, namely: tool life, roughness of the workpiece steel and grey cast iron materials and observations of cutting edge after work were also presented.

Section snippets

Preparation and testing methods

Si3N4-graphene composites were prepared using commercial powders: submicron silicon nitride (0.5–0.8 μm, Grade M11 of H.C. Starck) and nanometric graphene (average flakes thickness 8 nm, 20–30 monolayers with average particles size 550 nm, Grade AO-2 of Graphene Laboratories). To activate the sintering the aluminum nitride (0.8–1.8 μm, Grade C of H.C. Starck) and yttria (0.5–0.8 μm, Grade C of H.C. Starck) powders, with the 2.5 wt% and 4 wt%, respectively, were used. The graphene addition to the

Material densification

The materials obtained by hot pressing method showed very high densification. The relative density for the graphene content of up to 2 wt% was almost 100% of theoretical one calculated on the basis of phase composition determined by the Rietveld method. For higher quantities of graphene the relative density was higher than 98%.

Phase and microstructural analysis

The phase qualitative and quantitative analyses are presented in Table 1. These analyses were made only to describe the phase composition of the composites’ matrix.

The

Conclusion

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    The analysis of Raman spectra indicates graphene presence in sintered bodies.

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    Microstructural observations, ultrasonic measurements and hardness tests confirm the anisotropy of the Si3N4-graphene composite caused by applied pressure during the hot-pressing process.

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    The addition of the graphene additive above 2 wt% causes a significant decrease in mechanical properties of HP obtained composites.

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    A graphene content of more than 2 wt% leads to an increase in material wear.

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    The Si3N4-graphene cutting

Acknowledgements

The study constitutes a part of the project no. GRAF-TECH/NCBR/03/05/2012 “Ceramic composites with graphene content as cutting tools and device parts with unique properties”.

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