Tribological behaviour of Si3N4–BN ceramic materials for dry sliding applications

The main objective of this paper is to help on the clarification of the lack of consensus in the bibliographic data concerning the tribological behaviour of Si3N4–BN omposites. Unlubricated sliding tests by pin-on-disc were carried out with three grades of composite materials with 10, 18 and 25 vol.% of BN. The addition of BN to the Si3N4 matrix resulted in a slight reduction of the friction coefficient, which decreased from 0.82 for monolithic Si3N4 to 0.67 for Si3N4–10%BN materials. Wear coefficients (K) were above 10−5 mm3 N−1 m−1 for all materials tested and increased sharply with increases in BN volume fraction greater than 10%, e.g. K ∼ 10−3 mm3 N−1 m−1 for Si3N4–25%BN. The lowest values of friction and wear coefficients were obtained when the composites were tested with the BN platelets oriented parallel to the sliding direction. The morphological study of the worn surfaces revealed generalised brittle intergranular icrocracking at the dispersoid/matrix interface as the main wear mechanism. Under the experimental conditions of this study, the formation of stable protective layers of the soft lubricious oxide H3BO3, or the solid lubricant BN·H2O, was not observed

Functionality diagrams for hybrid mechanical seals with silicon nitride rings

Ring-on-ring tribological experiments were performed with hybrid mechanical seals with silicon nitride (Si3N4) rings. The K x PV product, where K is the wear coefficient, P the sealing pressure, and V the linear speed, is proposed as a novel parameter to characterize the total working range of a mechanical seal system, with the advantage of directly indicating the thickness reduction for a certain time of service. The KxPV criterion is represented in a map form called ‘‘functionality diagram.’’ The rotary Si3N4 rings show the better behavior (KxPV=0.05 µm/h) against WC–Co mating faces

Si3N4 and Si3N4/SiC composite rings for dynamic sealing of circulating fluids

Silicon nitride (Si3N4) and silicon itride/silicon carbide (Si3N4/30 wt.% SiC) seal rings were tested as self-mated pairs and dissimilar sealing systems against grey cast iron. The tribological experiments were conducted in a ring-on-ring tribometer at V ≈ 4ms−1 of linear speed, in the range of 0.3–1 kN of applied load, under a pressure of 2 × 105 Pa of a mixture of 20 vol.% of hydrogen peroxide in deionised water, which gives an effective pressure (P) between 0.2 and 1.3MPa. The homologous tribosystems are ineffective to seal due to catastrophic abrasive wear. The system with better performance is the dissimilar pair Si3N4/grey cast iron, that presented wear coefficient values of K = 4.2 × 10−8 mm3 N−1 m−1 for the ceramic ring and K = 1.3 × 10−7 mm3 N−1 m−1 for the metallic ring, in tests driven for about 60 h and 890 km, with full sealing. An amorphous passivating film of silica protects the ceramic surface. This system still presents an excellent combination K × PV ≈ 0.1mh−1, this product being a measure of the surface loss of a machine component for a given time of service

Silicon-incorporated diamond-like coatings for Si3N4 mechanical seals

Amorphous silicon carbide (a-SiC) and silicon-incorporated diamond-like carbon films (DLC-Si) were evaluated as protective and friction reduction coatings onto Si3N4 rings. Unlubricated tribological tests were performed with a pin-on-disk apparatus against stainless steel pins with loads ranging from 3 N to 55 N and sliding velocities from 0.2 m/s to 1.0 m/s under ambient air and 50-60% relative humidity. At the lowest loads, a-SiC coatings present a considerable improvement with respect to the behavior of uncoated disks since the friction coefficient is reduced to about 0.2 and the system is able to run stably for thousands of meters. At higher loads, however, a-SiC coatings fail. DLC-Si coated rings, on the other hand, presented for loads up to 10 N a steady state friction coefficient below 0.1 and very low wear rates. The lowest steady-state mean friction coefficient value of only 0.055 was obtained with a sliding velocity of 0.5 m/s. For higher loads in the range of 20 N the friction coefficient drops to values around 0.1 but no steady state is reached. For the highest loads of over 50 N a catastrophic behavior is observed. Typically, wear rates below 5 x 10-6 mm3/N.m and 2 x 10-7 mm3/N.m were obtained for the ceramic rings and pins, respectively, with a load of 10 N and a sliding velocity of 0.5 m/s. Analysis of the steel pin contact surface by SEM-EDS and Auger spectroscopy revealed the formation of an adherent tribo-layer mainly composed by Si, C and O. The unique structure of DLC-Si films is thought to be responsible for the formation of the tribo-layer

Integrated Taguchi-simulated annealing (SA) approach for analyzing wear behaviour of silicon nitride

In this study, the integrated Taguchi-simulated annealing (SA) approach is applied to examine the wear behaviour of silicon nitride (Si3 N4)-hexagonal boron nitride (hBN). Wear tests for Si3N4 -hBN composite versus steel (ASTM 316L) disc were carried out for a dry sliding conditions in a so-called pin-on-disc arrangement. The tests were realized at % volume of hBN 0, 4, 8, 12, 16 in Si3 N4 under the loads of 5, 10, 15, 20, 25 N. The wear rate (WR) was analyzed using Taguchi –signal to noise ratio approach with the aim of finding optimal combination of load and % volume of hBN in Si3N4 . By applying the analysis of variance, it was also found that the greatest impact on wear rate has interaction of load and % volume of hBN with percentage effect of 51.89%, then % volume of hBN with 35.04% and load with 13.06%. The experimental results are further utelized to develop the second-order, linear mathematical model. Further, this model is processed with simulated annealing (SA) to find the optimal combination of load and % volume of hBN to minimize wear rate. Combined Taguchi-SA approach was successfully used to predict the optimal combination of load and % volume of hBN in Si3 N4 to minimize wear rate of Si3 N4 . The dominant wear mechanism is adhesive wear as confirmed by scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS)

Optimization of wear loss in silicon nitride (Si3N4)–hexagonal boron nitride (hBN) composite using DoE–Taguchi method

Introduction The contacting surfaces subjected to progressive loss of material known as ‘wear,’ which is unavoidable between contacting surfaces. Similar kind of phenomenon observed in the human body in various joints where sliding/rolling contact takes place in contacting parts, leading to loss of material. This is a serious issue related to replaced joint or artificial joint. Case description Out of the various material combinations proposed for artificial joint or joint replacement Si3N4 against Al2O3 is one of in ceramic on ceramic category. Minimizing the wear loss of Si3N4 is a prime requirement to avoid aseptic loosening of artificial joint and extending life of joint. Discussion and evaluation In this paper, an attempt has been made to investigate the wear loss behavior of Si3N4–hBN composite and evaluate the effect of hBN addition in Si3N4 to minimize the wear loss. DoE–Taguchi technique is used to plan and analyze experiments. Conclusion Analysis of experimental results proposes 15 N load and 8 % of hBN addition in Si3N4 is optimum to minimize wear loss against alumina

Compósitos cerâmicos BN-Si3N4 de elevada resistência ao desgaste para material de corte

Sem resumo disponível.Mestrado em Engenharia de Materiai

Comportamento tribológico de anéis vedantes compósitos de nitreto/carboneto de silício

Os anéis de empanque asseguram a vedação de veios em sistemas de circulação de fluidos de natureza diversa sendo sujeitos a solicitações drásticas de natureza tribológica, com componentes mecânica e química por vezes extremamente agressivas. Os materiais cerâmicos são por isso uma classe em desenvolvimento para este tipo de aplicação. Neste trabalho fabricaram-se e caracterizaram-se sob o ponto de vista tribológico, anéis de nitreto de silício (Si3N4) e de compósitos nitreto de silício/carboneto de silício (Si3N4/SiC). O Si3N4, de entre os cerâmicos técnicos, é um dos materiais com melhores propriedades mecânicas, apresentando os corpos densos desenvolvidos neste trabalho valores de dureza na gama 14-16 GPa, de tenacidade à fractura entre 6 e 8 MPa· m1/2, de resistência à flexão aproximadamente de 800 MPa e módulo de Young de 280 GPa. A adição de SiC na forma de partículas à matriz de Si3N4 até 30% em peso, incrementou moderadamente este conjunto de propriedades. Os ensaios tribológicos foram efectuados num tribómetro anel/anel que simula, pela forma dos provetes, fluido de trabalho e parâmetros de operação, as condições de serviço dos sistemas de anéis vedantes. Ensaiaram-se pares homólogos de anéis cerâmicos e sistemas convencionais de metal duro e ferro fundido cinzento, sendo estes dois últimos usados para comparação. Testaram-se ainda pares cruzados cerâmico/metal duro e cerâmico/ferro fundido cinzento. Os parâmetros de ensaio situaram-se nas gamas de velocidade de rotação 1000-2000 rpm (velocidade linear, V, 2-4 m/s), carga normal total aplicada entre 0,3 e 1,3 kN (pressão efectiva, P, de 0,2 a 1,2 MPa), sob uma pressão de 2 bar de uma mistura de 20% de peróxido de hidrogénio em água desionizada. O sistema com melhor desempenho é o par vedante matriz-Si3N4/ferro fundido cinzento, que apresentou coeficientes de desgaste finais de Kd=4,2´10-8 mm3N-1m-1 para o anel cerâmico e Kd=1,3´10-7 mm3N-1m-1 para o anel metálico, em testes conduzidos por cerca de 100 horas e 1300 km, com vedação plena. A superfície cerâmica é protegida por um filme passivante amorfo de sílica. Este sistema apresenta ainda uma excelente combinação Kd×PV, produto que é proposto no presente trabalho como uma medida do volume de desgaste durante um determinado tempo de serviço. O Si3N4 tem um peso específico cinco vezes menor que o do metal duro, potenciando o recurso à sua aplicação em anéis de grande dimensão por questões energéticas. Os compósitos Si3N4/SiC não revelaram diferenças significativas face ao cerâmico monolítico de Si3N4. Ambos os materiais apresentaram elevado desgaste na situação de pares homólogos, ao contrário do bom comportamento revelado pelo metal duro e pelo ferro fundido cinzento.Mechanical rings are used to assure the sealing of shafts in systems of circulation of fluids. Hence, they are subjected to drastic tribological solicitations, namely in very aggressive chemical environments. Ceramics are thus a class of materials in development for this kind of application. In this work, silicon nitride (Si3N4) and silicon nitride/silicon carbide (Si3N4/SiC) rings were manufactured and characterized under the mechanical and tribological point of view. The Si3N4, among other technical ceramics, is one of the materials with better mechanical properties, the developed materials in this work presenting values of hardness in the range 14-16 GPa, fracture toughness between 6 and 8 MPa· m1/2, flexural resistance of 800 MPa and Young’s modulus of 280 GPa. The addition of SiC particles to the Si3N4 matrix up to 30% in weight, slightly increased these properties. The tribological experiments were conducted in a ring-on-ring tribometer that simulates the service conditions of such systems of seal rings, considering the samples geometry, the work fluid and the operation parameters. Homologous pairs of ceramic rings and conventional systems like hard metal and grey cast iron were studied for comparison. Also, dissimilar systems ceramic/hard metal and ceramic/grey cast iron were characterised. The test parameters were kept in the ranges of 1000-2000 of rotational speed (linear speed, V, 2-4 m/s), 0.3- 1.3 kN of total applied normal load (effective pressure, P, between 0.2 and 1.2 MPa), under a pressure of 2 bar of a mixture of 20% of hydrogen peroxide in deionised water. The system with better performance is the dissimilar pair ceramic matrix/grey cast iron, that presented wear coefficient values of Kd=4.2´10-8 mm3N-1m-1 for the ceramic ring and Kd=1.3´10-7 mm3N-1m-1 for the metallic ring, in tests driven for about 100 hours and 1300 km, with full sealing. The ceramic surface is protected by an amorphous passivating film of silica. This system still presents an excellent combination Kd´PV, this product being proposed in the present work to be a measure of the wear volume for a certain time of service. The Si3N4 ceramic has a specific weight five times lesser than the one of the hard metal, this characteristic being a supplementary benefit for its application as large diameter rings giving energy cost gains. The composites Si3N4/SiC didn't reveal significant differences towards the monolithic Si3N4 ceramics with respect to the tribological behaviour. Both materials showed severe wear when used as homologous pairs, unlike the good behavior revealed by the hard metal and the grey cast iron iron systems.Programa Doutoral em Ciência e Engenharia de Materiai

Communication Si 3 N 4 and Si 3 N 4 /SiC composite rings for dynamic sealing of circulating fluids

Abstract Silicon nitride (Si 3 N 4 ) and silicon nitride/silicon carbide (Si 3 N 4 /30 wt.% SiC) seal rings were tested as self-mated pairs and dissimilar sealing systems against grey cast iron. The tribological experiments were conducted in a ring-on-ring tribometer at V ≈ 4 m s −1 of linear speed, in the range of 0.3-1 kN of applied load, under a pressure of 2 × 10 5 Pa of a mixture of 20 vol.% of hydrogen peroxide in deionised water, which gives an effective pressure (P) between 0.2 and 1.3 MPa. The homologous tribosystems are ineffective to seal due to catastrophic abrasive wear. The system with better performance is the dissimilar pair Si 3 N 4 /grey cast iron, that presented wear coefficient values of K = 4.2 × 10 −8 mm 3 N −1 m −1 for the ceramic ring and K = 1.3 × 10 −7 mm 3 N −1 m −1 for the metallic ring, in tests driven for about 60 h and 890 km, with full sealing. An amorphous passivating film of silica protects the ceramic surface. This system still presents an excellent combination K × PV ≈ 0.1 m h −1 , this product being a measure of the surface loss of a machine component for a given time of service