Materials and Processing Issues Associated With Seal Coating Development

Transcription

1 Materials and Processing Issues Associated With Seal Coating Development J. D. Meyer and W. Y. Lee Department of Chemical, Biochemical, and Materials Engineering Stevens Institute of Technology 23 rd Annual Cocoa Beach Conference & Exposition Cocoa Beach, FL January 25, 1999

2 Acknowledgments Current research sponsored by Brad Beardsley at Caterpillar, Inc. The results from the chloride-based Al 2 O 3 coating work were obtained by W.Y. Lee at Oak Ridge National Laboratory under the sponsorship of Ray Johnson, Heavy Vehicle Propulsion System Materials Program, DOE Office of Transportation Technologies.

3 Rationale for Seal Coating Thermal Barrier Coatings (TBCs) are being considered to improve diesel engine efficiency CeO 2 -stabilized ZrO 2 (CSZ) prepared by air plasma spray (APS) provides thermal insulation of diesel components APS-CSZ is made porous for strain tolerance and enhanced thermal insulation Unexpectedly, testing at Caterpillar revealed a decrease in engine efficiency when components were coated with a TBC One possible reason may be the porosity of the TBC, which is suspected to entrain fuel from the combustion chamber prior to ignition [B. Beardsley, 1990] Thin seal coating? Fuel Vapor Porous CSZ Graded NiCrAlY-CSZ NiCrAlY Cast Iron Diesel Component

4 This Project Explores the Feasibility of Sealing the Porous TBC Surface by Applying a Seal Coating Materials Criteria Non-porous and impermeable Good adhesion to CSZ High thermal stability No debit to CSZ strain tolerance Resistance to erosion and wear Processing Criteria Processing temperatures below 500 C to avoid tempering of iron components Conformal coating on complex, porous TBC surface 12.5 µm Surface morphology of APS-CSZ (as received)

5 Coating Material Selection

6 Candidate Seal Coating Materials Were Screened Without Cast Iron Substrate Free-standing APS-CSZ coupons (1x1cm) were coated with: MATERIAL CTE (x10-6 /K) MODULUS (GPa) α-al 2 O Al 2 O 3 2SiO SiO 2 (fused) CSZ (substrate) ~10 ~200 Si (substrate) High-temperature chloride-based CVD processes were used: 2AlCl 3 + 3CO 2 + 3H 2 Al 2 O 3 + 6HCl + 3CO (1050 C) SiCl 4 + 2CO 2 + 2H 2 SiO 2 + 4HCl + 2CO (1050 C) Thermally cycled to 1150 C in air to assess seal coating/csz stability 3Al 2 O 3 2SiO 2 and SiO 2 spalled during thermal cycling, probably due to CTE mismatch with respect to CSZ

7 Al 2 O 3 Seal Coating (from Chloride Process) Was Uniform, Conformal, and Thermally Stable As coated: After 49 1-h. Cycles to 1150 C. 2.5 µm 2.5 µm Epoxy Mount Coating 12.5mm Substrate Intensity a After thermal cycles (mostly α-al 2 O 3 ) As-deposited As deposited (mostly θ-al 2 O 3 ) a: a-al 2 O θ 3 a a a θ θ 2.5mm q

8 Screening Results Suggest That Metastable Al 2 O 3 Seal Coating May Be Useful If It Can Be Prepared at T < 500 C Free-standing CSZ without seal coating Free-standing CSZ coated and cycled

9 MOCVD Al 2 O 3 Coating Synthesis

10 Al(acac) 3 and H 2 O Were Selected to Prepare Low-temperature Al 2 O 3 Seal Coating Major reasons: Decomposes readily (well below 500 C) CH 3 Low toxicity and cost Relatively moisture-insensitive C O Stable compound at room temperature H C Al Some carbon contamination observed C O Inclusion of water vapor appears to help eliminate carbon contamination [J.S. Kim et al., 1993] CH 3 3

11 Cold-wall Al 2 O 3 MOCVD System Constructed Ar Mass Flow Controller 130 C Al(acac) 3 Substrate Inconel Heater Chamber Mass Flow Controller 26 C H 2 O Vent Pressure Transducer Throttle Valve Isolation Valve Pump

12 Uniform and Conformal MOCVD Al 2 O 3 Seal Coating Was Prepared at 500 C 10 µm Al 2 O 3 on Silicon 10mm Al 2 O 3 on CSZ

13 Thermal Stability of MOCVD Al 2 O 3 Coating

14 Thinner Al 2 O 3 Coating on Silicon Did Not Spall upon Annealing 2.25 µm 800 C 2.25 µm 900 C 10 µm As coated 250 µm 250 µm Non-spalled Al 2 O µm 1100 C 0.16 µm 1100 C Al 2 O 3 Coating Silicon 250 µm 250 µm

15 Crystallization of Al 2 O 3 Occurred Relatively Rapidly (<20 Hours) at 700 C to 1200 C q, β, S 1200 ºC a Not Indexed a q, d, κ a, q, S Not Indexed 800 ºC 26 ºC a: α-al 2 O 3 S: Al 2 Si 4 O 10 2q (degrees) Not much Al 2 O 3 remained on the substrate for XRD analysis

16 Al 2 O 3 on Constrained CSZ/Iron Substrate Cracked Upon Annealing 10x 10mm 10mm As coated 900 C 1mm Al 2 O 3 Seal Coating (2 µm thick) 10x Porous CSZ Cast Iron Substrate 10mm 1000 C 1mm

17 Comparison & Summary

18 Annealing of MOCVD Al 2 O 3 Leads to Inadequate Adhesion and Sealing Considerable spallation on silicon with 2.25 µm coating CTE mismatch Volume shrinkage due to crystallization ( > ~9%) Sub-micron coatings on silicon were less susceptible to spallation Adhered on CSZ, but coating cracked as crystallization occurred Less CTE mismatch with CSZ than with Si Better adhesion may be due to mechanical interlocking at CSZ/coating interface Volume shrinkage still significant ( > ~ 9%)

19 As prepared: Comparison of Chloride-based Al 2 O 3 vs. MOCVD Al 2 O 3 Chloride-based Al 2 O 3 Conformal, mostly metastable (θ) MOCVD Al 2 O 3 Conformal, amorphous Thermally annealed: Retained adhesion & no cracking Severe cracking, despite adhesion Crystallization: θ-al 2 O 3 α-al 2 O 3 ( V ~ -9%) Amorphous metastable, α-al 2 O 3 ( V > -9%) Possible C & H impurities: Highly unlikely Possible, but minimized Quality of sealing: Sufficient Insufficient

20 Conclusions Chloride-based Al 2 O 3 coating deposited at 1050 C was mostly θ-al 2 O 3 Sealed the porous surface, although it transformed from θ- to α-al 2 O 3 MOCVD Al 2 O 3 coating could be prepared at 500 C, but was amorphous Adhered to CSZ upon annealing, but cracked extensively Metastable Al 2 O 3 coating (e.g., θ-al 2 O 3 ) and appropriate processing modifications may be required