Strength and fatigue of NT551 silicon nitride and NT551 diesel exhaust valves

Abstract

Executive SummaryThe inert strength and fatigue performance of a diesel engine exhaust valve made from silicon nitride (Si~Nq) ceramic were assessed. The Si~Na characterized in this study was manufactured by Saint Gobain / Norton Industrial Ceramics and was designated as NT551. The evaluation was performed utilizing a probabilistic life prediction algorithm that combined censored test specimen strength data with a Weibull distribution function and the stress field of the ceramic valve obtained from finite element analysis. The major assumptions of the life prediction algorithm are that the bulk ceramic material is isotropic and homogeneous and that the strength-limiting flaws are uniformly distributed.The results from mechanical testing indicated that NT551 was not a homogeneous ceramic and that its strength was a function of temperature, loading rate, and machining orientation.Fractographic analysis identified four different failure modes; 2 were identified as inhomogeneities that were located throughout the bulk ofNT551 and were due to processing operations. The fractographic analysis concluded that the strength degradation of NT551observed from the temperature and loading rate test parameters was due to a change of state that occurred in its secondary phase.. Pristine and engine-tested valves made from NT551 were loaded to failure and the inert strengths were obtained. Fractographic analysis of the valves identified the same four failure mechanisms as found with the test specimens.The fatigue performance and the inert strength of the Si~NAvalves were assessed from censored and uncensored test specimen strength dat~respectively. The inert strength failure probability predictions were compared to the inert strength of the Si~Nqvalves.. . . The inert strength failure probability predictions were more conservative than the strength of the valves. The lack of correlation between predicted and actual valve strength was due to the . nonuniform distribution of inhomogeneities present in NT551. For the same reasons, the predicted and actual fatigue performance did not correlate well.

Evaluation of Silicon‐Nitride Ceramic Valves

“…7 was attributed to an uneven distribution of the second-phase component in the Si 3 N 4 material that was not considered to be detrimental to the strength. 7,8 The detected high damage levels in the valve-head surfaces, however, would likely reduce valve strength.…”

“…[1][2][3][4] The benefits offered by ceramics over existing metals are attributed to the favorable material properties of ceramics, including lower density, superior strength at elevated temperatures, higher wear and corrosion resistance, and lower thermal expansion. [5][6][7][8][9][10][11] Advanced ceramics may also become necessary for new engine systems that require the use of light-weight components in higher temperatures to achieve further engine efficiency and emission reduction to meet ever-stringent environmental regulations. However, several barriers exist for the wide use of ceramics; the two major barriers are cost and reliability.…”

“…[19][20][21] The strength and dynamic fatigue properties of several candidate commercial Si 3 N 4 were studied extensively. 8,[22][23][24] These studies showed that the strength is dependent on operation temperature, fatigue (slow crack growth), extrinsic flaws (e.g., machining damage), and intrinsic flaws (second phase stability and uniformity, porosity, etc). However, it was confirmed from failure probability predictions that the strength range of these Si 3 N 4 is more conservative than required for engine valves.…”

“…For high-strength Si 3 N 4 , fracture-initiation flaws may be smaller than 50 mm in size and are located on or beneath the surface. 7,8,28 The fracture origins normally consist of microstructural irregularities that act as a stress concentration from the applied load. These flaws may be intrinsic to the material such as agglomerates, inclusions, and pores distributed throughout the volume, or extrinsic due to component manufacturing and service such as machining or impact damage.…”

“…These flaws (and their distribution) may significantly degrade the strength and make it difficult to predict the lifetime of ceramic components. [7][8]29 To assure the quality and reliability of ceramic valves, nondestructive evaluation (NDE) methods are required to detect and characterize surface/subsurface flaws in the entire valve surface. NDE methods can therefore be used to assess the cost-effective manufacturing process during valve production, and to investigate the reliability and durability of ceramic valves under engine applications.…”