degradation of silicon nitride glow plugs in various environments part 3: ngdi engine

Degradation of Silicon Nitride Glow Plugs in VariousEnvironments Part 3: NGDI Engine

Carmen Oprea,* Frankie Wong, Hamed Karimi Sharif, and Tom Troczynski

Materials Engineering, University of British Columbia, Vancouver, BC V6T 1Z4, Canada

Colin Blair and Alan Welch

Westport Innovations Inc., 1691 West 75th Ave., Vancouver, BC V6P 6P2, Canada

Providing a reliable hot surface ignition system (glow plug, GP) for natural gas-direct injection engines is challenging.This paper presents experimental results of aging all-ceramic Si3N4-based GPs in an engine, continuing previously published

results on electric and gas burner rigs. The microstructural modifications of the ceramic heater, leading to degradation andultimately to failure, are effected by four synergistic mechanisms: electrical, chemical, mechanical, and thermal. GP lifetime inengine follows the general Arrhenius law, with activation energy of 5.2 eV, (vs 9.1 V on burner rig and 13.8 eV on electricrig, as reported previously), suggesting additional factors contributing to GP failure in the engine.

Introduction

Westport Innovations Inc., Vancouver, BC Canadadevelops advanced natural gas and direct injection(NGDI) engines, including hot surface ignition system.All-ceramic glow plugs (GP) based on silicon nitrideappear to be promising candidates for such a system,but they must be fully characterized before introduction

to commercial engines. We have already reported onGP damage modes in simulated environments in elec-tric and gas burner rigs in previous works.1,2 However,it is well known that the harsh dynamic environmentof a NGDI engine introduces additional specific issuescontributing to the GPs degradations, including cyclicelectrical loads, high-velocity gas motion (up to 400 m/s),mechanical vibrations, cycling reducing/oxidizing condi-tions, and thermal cycling with air/fuel introductionfollowed by combustion for each engine cycle. It wasanticipated that high-frequency high-pressure gas and

*[email protected]

© 2011 The American Ceramic Society

Int. J. Appl. Ceram. Technol., 9 [2] 272–279 (2012)DOI:10.1111/j.1744-7402.2011.02724.x

reducing/oxidizing conditions swings should significantlyaffect the pattern of surface corrosion of GPs. Althoughsimilar glow plugs have been successfully used in conven-tional Diesel engines, the operating environment is muchless demanding, that is temperature ~ 1000°C or less,and time up to only about 5 min during cold enginestartup. In the high compression ratio NGDI engine, theGPs have to perform continuously at about 1300°C torapidly drive the ignition process, as NG has a higheractivation energy than Diesel fuel. The primary objectiveof this work is to evaluate how this type of GPs per-forms in this much harsher environment. The reader isreferred to 1, 2 for details of the background of thisresearch, including the internal architecture of the GPsand the composition of the ceramic heater. Briefly, theall-ceramic part of the glow plug includes two U-shapedWC-based electric conductors embedded in Si3N4-Yb2O3

insulating rod (4.2 mm in diameter). One of the mostsignificant GP damage mechanisms reported previouslyfor both burner and electrical simulation rigs includesredistribution of Yb3+ additive ions under the influenceof dc electric field at high operating temperature. Theredistribution triggers several other damage phenomenawhich eventually lead to catastrophic failure of the glowplugs.1, 2 Catastrophic damage refers to a sudden end ofthe ability of the GP to ignite fuel

Experimental Procedures

Twelve commercially available GPs manufacturedby Kyocera Corporation (Kyoto, Japan), with 4.2 mmdiameter of ceramic heater, were tested at Westport in afour cylinder 5.2 L Isuzu diesel engine with 18:1 com-pression, and using stratified mixture of injected natural

gas in air. Four GPs were tested in the European Tran-sient Cycle (ETC), which is a light-duty drive cycle,with low engine speed and load (average 1635 rpm and165 Nm, respectively). The testing parameters and asummary of the results in ETC are presented in Table I.Due to time constrains, eight more glow plugs weretested under Steady State (SS) engine cycles, at averageengine speed of 2000 rpm and average load of450 Nm; the variable parameters were the applied volt-age and duration of testing. The Steady State is anaccelerated test, which ran the glow plugs at higher tem-peratures and considerably higher engine load (~90% ofmaximum load) and speed than normally experiencedin the engine. The testing parameters and a summary ofthe results in SS are presented in Table II.

The glow plug voltage and currents were recordedusing the data-logging feature of the GP control unit.The GP surface temperature was estimated from thevoltage values assigned through the GP control unitand the characterization of the GPs before and aftertesting, when glow plug resistance, diameter, mass andsurface temperature were determined. During character-ization, the GP surface temperature was read with a 1-color Ircon optical pyrometer, which averages the tem-perature read over the entire spot and it was used toscan the pin and find the hottest location; as detailedin,2 its maximum reading error was ± 10°C. The glowplugs were either removed while they still performednormally, or were tested until failure. When the GPsfailed on the electric rig 1 or gas burner rig,2 one oreven both resistor loops were physically destroyed; how-ever, in the engine, failure means the inability to fur-ther maintain ignition of the fuel, regardless of thedegree of internal degradation in the ceramic heater,hence this could be considered a partial failure. None-

Table I. GP Testing Parameters and Summary of Results in ETC (1635 rpm, 165 Nm)

SpecimenVoltage(V)

Surfacetemperature* (°C)

Testduration

(h)Failed orrunning†

Min Yb‡

(wt%)Max Yb‡

(wt%)Mass loss rate§

(mg/h)

ETC1 9.50 1270 847.4 R 1.88 9.35 0.171ETC2 9.50 1300 992.4 R 1.83 9.65 0.191ETC3 9.50 1315 911.0 R 2.19 9.81 0.221ETC4 11.00 1520 64.0 R 2.15 10.16 0.250

*Estimated from characterization values and applied voltage.†At the end of the testing period.‡Elemental Yb content determined by EDS through Si3N4 insulator cross-sections at the hot spot.§Calculated from the weights of the entire GPs before and after testing.

www.ceramics.org/ACT Degradation of Silicon Nitride Glow Plugs - Part 3 273

theless, it is believed that such a condition is fairly closeto the total heater failure.

At the end of the engine testing, cross-sections of theceramic pins were prepared in the manner described indetails in,1,2 that is sectioned and characterized at the“hot spot,” where the highest temperature was read dur-ing characterization. They were studied by SEM/EDS,using a Hitachi S 3000-N (Hitachi, Tokyo, Japan)Scanning Electron Microscope with an Electron Disper-sive X-Ray Spectroscopy detector in the Low Vacuum –Variable Pressure mode with Back Scattered Electronmethod. The variation of the sintering additive cation(Yb3+) through the intergranular phase of the Si3N4 insu-lator was determined by the concentration by weight (wt%) of the elemental ytterbium as detected by EDS on thecross-sections. The standard deviation values were 1.4–2.0% and the maximum error for the calculated Yb was± 2.6 wt%. All the micrographs presented are of thecross-sections through the ceramic pins at the hot spot.

Results and Discussion

At the conclusion of the engine tests, there weresurface deposits on the first 10–12 mm from the frontend of all-ceramic pins, that is on the hot zone. Simi-larly to the specimens tested on the electric rig and onthe burner rig, the surface deposit was mostly ytterbiumdisilicate precipitated on the layer of silica formed as aresult of silicon nitride oxidation. In this case, however,the silicatic deposit was even less protective toward the

silicon nitride, due to the cyclic presence of reducingconditions, which would reduce the SiO2 to the volatileSiO, in addition to the possible erosion by thehigh-velocity gas movement within the cylinder. Thespecimens tested in this study were manufactured onan industrial scale and as such, there are small butinherent variations in their properties; for example, the9.5 V applied to three of the GPs tested in the ETCcycle yielded temperatures between 1270 and 1315°C,due to variation in their resistivity (Table I).

In the light-duty cycle testing, three specimens(ETC1, 2, 3) were removed from the engine after run-ning for 847.4–992.4 h, when they were still fullyfunctional, that is they still provided ignition of thefuel. At these temperatures (1270–1315°C, normal forthis type of engine), the internal structure of the cera-mic pin did not degrade much during this set of exper-iments. The pattern of Yb3+ ions migration was similarto the electric rig and burner rig, but in this case, dueto erosion by the high pressure/velocity hot gases, therewas loss of mass from the outside of the ceramic pin,in contrast to the mass gain observed on the rigs.1, 2

Low magnification micrographs of the cross-sections atthe hot spots through the Si3N4 pins of the GPs testedfor different durations in ETC are shown in Fig. 1.The loss of uniformity of the sintering additive ionYb3+ and its redistribution in the intergranular phase ofSi3N4 is apparent in all images by the darker-contrastedareas, with a lower concentration of Yb, and thelighter-contrasted areas, with higher Yb (Fig. 2). As

Table II. GP Testing Parameters and Summary of Results in Steady State (2000 rpm, 450 Nm)

SpecimenVoltage(V)

Surfacetemperature* (°C)

Testduration

(h)Failed orrunning†

Min Yb‡

(wt%)Max Yb‡

(wt%)Mass loss rate§

(mg/h)

SS1 12.9 1450 4.8 R 1.8 11.7 �3.083SS2 12.9 1450 50.1 F 0.79 16.8 0.952SS3 12.8 1450 50.1 F 0.75 14.0 1.07SS4 13.3 1475 29.7 F 1.76 14.8 0.892SS5 13.7 1500 8.90 F 1.09 12.1 0.551SS6 14.3 1525 4.80 F 0.91 10.0 0.063SS7 14.3 1550 8.90 F 0.00 22.8 0.337SS8 14.3 1550 8.90 F 0.00 24.4 1.135

*Estimated from characterization values and applied voltage.†At the end of the testing period.‡Elemental Yb by EDS through Si3N4 insulator in cross-sections at the hot spot.§As calculated from the weights of the entire GPs before and after testing.

274 International Journal of Applied Ceramic Technology—Oprea, et al. Vol. 9, No. 2, 2012

determined by EDS, the insulator phase in the as-received GPs contains ~ 9 wt% Yb; in these specimens,Yb varies between 1.83 and 10.16 wt% (Table I); thisis the same range as on the burner rig, at the lowesttemperature used (1425°C) for only 10 h.2

Even though the silicatic deposit of Yb2Si2O7 hasformed on the surface of the ceramic pins (so thereshould have been a mass gain due to the ingress of oxy-gen from the environment), the overall mass balance atthe conclusion of testing was negative; the average massloss for these GPs was 0.171–0.250 mg/h (Table I).Material was lost from the surface of the pins, due toboth erosion by the hot high velocity gases, enginevibration, and complex chemical red-ox reactions. TheGPs placed in the engine’s cylinders were surroundedby perforated metal shields and there was visible loss ofmaterial corresponding to the positions of the holes inthe shields, where the velocity is the highest. The West-port CFD (computational fluid dynamic) analysis indi-cated that peak velocities of ~ 150 m/s can occur nearthe inlet holes and velocities in the 10 m/s range canoccur elsewhere inside the shield. For the GPs testedin ETC the mass loss rate slightly increased with tem-perature, however, for the GPs tested in SS conditionthe data points are too scattered and no meaningfulconclusion can be drawn (Fig. 3).

The GPs tested under the steady state (SS) weresubjected to higher voltage (12.8–14.3 V), hence tem-peratures, as well as to much harsher engine conditions(higher speed and load), as an accelerated test, due totime constrains (Table II). These GPs were tested fourat a time, in the four cylinders of the engine, and this-created some inconsistencies in the results, as differentcylinders have somewhat different air flow, fuel flow,injector orientation, and so forth, which in turn couldcreate differences in the wear rates of the GPs.

Micrographs of the cross-sections of representativespecimens are shown in Figs. 4–6. The glow plug SS1was removed from engine after 4.8 h, when it was stillfunctional, while all the other SS-series samples weretested to failure. Due to the higher applied voltage and

Fig. 1. Micrographs of cross-sections for specimens ETC2 andETC4; the (+) side is on the right.

Fig. 2. Micrographs of specimens ETC2 and ETC4: darker-contrasted Si3N4 areas indicate Yb

3+ migration away from theside of the resistors connected to the (+) pole.

Fig. 3. Mass loss rate variation with temperature for transientand steady state cycles.

Fig. 4. Micrographs of cross-sections for specimens at 1450°Cin SS: SS1 still working after 4.8 h; SS2 failed after 50.1 h.


higher temperatures, the degradation of the internalstructure occurred sooner and to a higher degree thanin ETC. Elemental ytterbium values ranged betweenminimum of 1.8 wt% and maximum of 11.7 wt% forSS1 and minimum 0 and maximum 24.4 wt% forSS8, compared to maximum ~ 55 wt% on the E-rigand ~ 48 wt% on the B-rig.1,2 This lower divergencebetween the minimum and maximum values indicatesthat in the engine, the electric field in the GP does nothave a dominant role in the deterioration process.

Figure 5 depicts the microstructure of the positive(+) side of the resistor loops at the hot spot for thesamples tested at 1450°C (SS1, 2, 3). The GP sampleSS1 that was tested for only 4.8 h and was still func-tional at the end of testing, showed very little degrada-tion of both the resistor and the surrounding Si3N4

insulator. The resistor of the failed SS2 sample wasprobably also in good condition (judging by the centralportion left in place), but the Si3N4 had lost its inter-granular phase in the vicinity, so part of the resistormaterial became dislodged during sample preparation.

The significant mass loss rate and diameter loss inthe ceramic heaters of the GPs that failed under ETC

and SS (Table II, Fig. 7) indicate that the erosion bythe hot gases is the dominant cause of the performancedegradation of the Si3N4-based GPs in the engine. Aswas reported elsewhere,2 in a natural gas burning rig,where GPs were only subjected to natural gas flame,the oxidation of silicon nitride ceramics resulted in anet weight gain in the GPs. Hence, a relatively signifi-cant mass loss in the engine tests can be attributed tothe erosion of the oxide scale due to the presence ofhigh-temperature and high-pressure combustion gas.This becomes more evident when comparing the rateof mass loss in two different SS and ETC testing con-ditions, that is the maximum mass loss rate of the cera-mic material in SS was 4.5 times higher than themaximum in ETC (1.135 mg/h for SS8 compared to0.250 mg/h for ETC4).

The influence of the engine cycle parameters andof the required heat energy for the ignition of fuel/airmixture on the internal degradation of the GPs isshown in Fig. 8. In ETC the voltage applied to reach1520°C was 11.0 V, whereas in SS, to reach a similartemperature (1525°C), 14.3 V had to be applied dueto high-velocity gas flow and significant heat dissipa-tion. The higher electrical field in SS led to fastermigration of the Yb3+ ions away from the (+) side ofthe resistor and through the bulk of the Si3N4 insula-tor. However, as discussed in previous studies,1 no cor-relation can be established between the redistribution of

Fig. 5. Micrographs of the center and ends of the (+) sideresistors and surrounding Si3N4 of sampled tested at 1450°Cin SS.

Fig. 6. Micrographs of cross-sections for samples failed in SS:SS5, after 8.9 h at 1500°C and SS8, after 8.9 h at 1550°C.

Fig. 7. Diameter loss variation of the GPs in ETC (top) andSS (bottom) with test duration.


sintering additives within the cross-section of ceramicheaters and the perceived weight gain of the GPsloaded by an electric field in ambient air. In fact, themigration of sintering aids mainly contributes to theloss of mechanical and electrical properties of ceramicheater. Also, in cases when the insulator structure wasvery cracked and porous allowing the oxidation ofWC-based conducting phase, tungsten was detected asWO3 on the surface and in the bulk of Si3N4 insula-tor.1 However, as mentioned earlier, in the engine, fail-ure is defined as the inability of the ceramic tip toprovide sufficient heat to ignite the natural gas/air mix-ture. For instance, the SS6 failed to maintain the igni-tion after only 4.8 h, although ETC4 was still runningat the end of the experiment, after 64 h. Therefore,extreme conditions under which at least one of the two

heating conductors were severely deteriorated ordestroyed were not observed in this study, even in theharsh SS engine environments.

On the electric rig (air exposure only), the degra-dation and failure occurred due to the redistribution ofthe sintering additive ions (Yb3+) under the influence ofthe electric field generated when the GPs were powered,and the oxidation of Si3N4.

1 On the burner rig (fuel+ air), additional factors contributed to the degradationprocess, namely the chemical interactions between theceramic and the combustion gases.2 To these, theengine adds cyclic oxidation/reduction and thermo-mechanical damage modes (pulsing flow, vibration,thermal shock), which accelerate the loss of ceramicmaterial from the outside surface, especially in the hot-test region. These synergistic factors cause the glowplugs to fail much faster in the engine, and throughdifferent mechanisms (Fig. 9).

On the test rigs, the maximum Yb concentration inthe cross-sections at the hot spot was determined fordifferent temperatures at failure.1,2 It was hoped thatthis could be used to develop an indicator to help pre-dict the GP life in engine, but this is not the case. Afterthe engine testing, the redistribution of the Yb throughthe intergranular phase of the insulator was found to bemuch less significant, with values much less divergentfrom the as-received, and inconsistent. During theengine testing of the glow plugs at Westport, a signifi-cant mass loss was however recorded, for which we canidentify the following concurring causes (a, b, c):

SiO2 Volatilization

The film of SiO2 formed by the inherent oxidationof the Si3N4 according to the reaction Si3N4 + 3O2?3SiO2 + 2N2 should protect the Si3N4 bulk from fur-

Fig. 8. Effect of the different voltages needed to reach the sametemperature (1520–1525°C) in SS (top) versus ETC (bottom).

Fig. 9. Micrographs of cross-sections at the hot spot for GPs failed at 1450°C on the simulating rigs (E-electric and B-burner) and inthe engine.


ther degradation in an oxidizing combustion environ-ment. However, if the oxidants’ pressure falls belowthat needed for a stable SiO2 film, gaseous SiO and N2

would form through the reaction Si3N4 + 1.5O2(g)= 3SiO(g) + 2N2(g).

3 Moreover, local fuel-rich areasoccur cyclically, with reducing components such as COand H2 (besides CH4), which would reduce the solidSiO2 scale to gaseous SiO, causing physical loss ofmass.4 Other gaseous Si-O-H phases could also form inthe presence of water vapor; most probable under theconditions in an internal combustion engine being Si(OH)4 through the reaction SiO2(s) + 2 H2O = Si(OH)4(g)

5,6

Erosion by the Hot High Velocity Combustion Gases

There was visible loss of the ceramic material,mostly corresponding to the positions of the holes inthe metallic shields surrounding the glow plugs, asdepicted for example in Fig. 1, ETC4. The silicaticscale formed on the surface is spalling off, exposingunprotected Si3N4 to oxidation/corrosion cycles, underthe very high gas velocity of up to 150 m/s.

Impurities in the Fuel

It is well known that even in small amounts,impurities such as SO2 and SO3, C, HCl, Na, vana-dates, atoms of transition metals, can have a deleteriouseffect on a silicon-based ceramic.7, 8

The time-to-failure of the GPs on the simulatingrigs and in the engine was used to determine the dura-

bility curves, as presented in Fig. 10. The time-to-failure t was expressed in the form of an Arrheniusrelationship, t ¼ Ae

EkT , where t is time-to-failure in

hours, A is a constant, E is the activation energy (eV),k is the Boltzmann constant (8.6171e-5 eV/K), and Tis the absolute temperature (K).

Extrapolating the regression line to 1300°C, whichis the operating temperature of the GPs in the engine,the expected life on the engine was ~1200 h, comparedto ~52,000 h on the burner rig and 20,000,000 h onthe electric rig. This was expected, as the engine intro-duces many supplemental modes of degradationthrough the thermal shock, cyclic oxidation/reduction,vibrations, pulsing flow, high velocity hot gases, and soforth, as described above. This value for the time-to-failure in engine was verified by the three GPs tested atthe normal operating temperatures, with 9.5 V and inthe ETC, at average 1635 rpm engine speed and165 Nm engine load. The GPs were removed from theengine cell at almost 1000 h and were still functional,ensuring reliable ignition of the fuel.

Using the Arrhenius relationship and the powerregression curves from Fig. 10, the activation energy(combined value for all operating failure mechanisms)was calculated at 5.2 eV, much lower than the 13.8 eVfor the E-rig and 9.1 eV for the B-rig, which indicatesthe additional failure mechanisms present in the engine.However, the slope of the Arrhenius curve C does notchange, which suggests that there is no change in thefailure mechanisms over the range of temperatures usedin the engine testing. The power regression curves didnot show a very good correlation coefficient(R2 = 0.774), due to the inherent variation in thetesting conditions, as the GPs were tested in differentcylinders, which means different deterioration rates.

The estimated 1200 h life in service on the engineis not acceptable for this application (minimum 5000 hand ideally 20,000 h are desired), so means to protectthe surface of the ceramic heater operating continuouslyat 1300°C in NGDI engine must be implemented.A variety of techniques to modify the Si3N4 surface byion implantation, coatings, hydrothermal surface treat-ment, have been studied in to improve the oxidationand corrosion resistance.9–14 Deposition of a mullite-mullite coating, followed by in-situ calcination by pow-ering the glow plug has been evaluated in our labs(Wong et al. [2010], unpublished data) and it looksvery promising, as tested in air. After 100 h at 1300°C,the layer of Yb2Si2O7 formed below the coating was

Fig. 10. Glow plug durability curves on the simulating rigsand in the engine.


about one-fourth of that developed on an uncoatedglow plug. This work is being submitted for publica-tion.

Conclusions

Some aspects of the Si3N4-based GP degradationin natural gas with direct injection (NGDI) engines arepresented. The testing of the GPs was conducted bothin the ETC cycle and on steady state rigs, for acceler-ated assessments. Based on the results of the acceleratedtests, the time-to-failure at 1300°C on the engine waspredicted to be about 1200 h, and the activationenergy for the dominant damage process was 5.2 eV.Both values are much lower than those estimated previ-ously on the gas burner rig and electric rigs (52,000 hand 20,000,000 h, respectively). This reflects the differ-ent failure mechanisms in different environments. Theengine is a much more complex environment ascompared to the electric and burner rigs, with at leastfour different modes of degradation contributingsynergistically to the failure of the GPs:

(1). Chemical degradation, through oxidation ofSi3N4 to SiO2, and reduction of the protectiveSiO2; migration of the sintering additive ionstoward the surface (which degrades the mechanicaland electrical properties of the ceramic heater);reactions with the impurities in combustion gases(which form soft silicates, which are easy toremove).

(2). Mechanical degradation: erosion by the high-pres-sure high-velocity (up to 150 m/s) combustiongases, leading to mass loss in the ceramic heater;cracking due to thermal cycling and the associatedthermal stress due to differences in thermal expan-sion coefficients of surface oxides and the basematerial.

(3). Thermal degradation: increased temperatureincreases the rate of chemical reactions at an expo-nential rate and is accelerated by the electricaldegradation of the conductive phase.

(4). Electrical degradation: the sintering additives ionsmigrate under the electrical field, leading todielectric breakdown of Si3N4 across the resistors;higher applied voltages accelerate this process.

Acknowledgment

Supported by the Natural Sciences and Engineer-ing Research Council (NSERC) of Canada.

References

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degradation of silicon nitride glow plugs in various environments part 3: ngdi engine

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