Fracture behavior and thermal durability of lanthanum zirconate-based thermal barrier coatings with buffer layer in thermally graded mechanical fatigue environments

The effects of buffer layer on the fracture behavior and lifetime performance of lanthanum zirconate (La2Zr2O7; LZO)-based thermal barrier coatings (TBCs) were investigated through thermally graded mechanical fatigue (TGMF) tests, which are designed to simulate the operating conditions of rotating parts in gas turbines. To improve the thermal durability of LZO-based TBCs, composite coats consisting of two feedstock powders of LZO and 8 wt% yttria-doped stabilized zirconia (8YSZ) were prepared by mixing different volume ratios (50:50 and 25:75, respectively). The composite coat of 50:50 volume ratio was employed as the top coat, and two types of buffer layers were introduced (25:75 volume ratio in LZO and 8YSZ, and 8YSZ only). These TBC systems were compared with a reference TBC system of 8YSZ. The TGMF tests with a tensile load of 60 MPa were performed for 1000 cycles, at a surface temperature of 1100 °C and a dwell time of 10 min, and then the samples were cooled at room temperature for 10 min in each cycle. For the single-layer TBCs, the composite top coat showed similar results as for the reference TBC system. The triple-layer coating (TLC) showed the best thermal cycle performance among all samples, suggesting that the buffer layer was efficient in improving lifetime performance. Failure modes were different for the TBC systems. Delamination and/or cracks were created at the interface between the bond and top coats or above the interface in the single-layer TBCs, but the TBCs with the buffer layer were delaminated and/or cracked at the interface between the buffer layer and the top coat, independent of buffer layer species. This study allows further understanding of the LZO-based TBC failure mechanisms in operating conditions, especially in combined thermal and mechanical environments, in order to design reliable TBC systems

Analytical model of IEEE 802.15.4 non-beacon mode with download traffic by the piggyback method

Abstract. We analyze the MAC performance of the IEEE 802.15.4 LR-WPAN non-beacon mode with the piggyback method in non-saturated condition. Our approach is to model a stochastic behavior of one device as a discrete time Markov chain. We propose an analytical model describing the download behavior of a device using piggyback method. We obtain the performance measures such as throughput, packet delay, energy consumption and packet loss probability of a device. Numerical results and simulation results show that the piggyback method which removes a backoff procedure in the backoff method can reduce the delay, loss probability and energy consumption compared with backoff method. Our results can be used to find the optimal number of devices with some constraints on packet delay and packet loss probability

Performance Analysis of IEEE 802.15.4 with Non-beacon enabled CSMA/CA

Abstract. This paper proposes an analytical model of IEEE 802.15.4, which is a standard toward low complexity, low power consumption and low data rate wireless data connectivity. In this paper, we concentrate on the MAC performance of the IEEE 802.15.4 LR-WPAN in a star topology with unslotted CSMA/CA channel access mechanism under non-saturated modes. Our approach is to model stochastic behavior of one device as a discrete time Markov chain model. We believe that many WSN applications would benefit from our analytical model because many applications in WSN generate traffic in non-saturated mode. We obtain five performance measures : throughput, packet delay, number of backoff, energy consumption and packet loss probability. Our results are used to find optimal number of devices satisfying some QoS requirements