Diamond-like carbon (DLC) coated ball bearings cut friction and abrasive wear in poor-lubrication conditions, but they do not automatically remove the need for grease. Hydrogen content, environment, speed and contamination load decide whether the coating acts as a solid lubricant on its own or runs paired with a conventional grease [S1][S2].
The 2025 Piotrowska comparative study on 100Cr6 steel versus PACVD-deposited a-C:H DLC explicitly tested both dry friction and Renolit UNI 3 grease-lubricated conditions on belt-conveyor bearing components, then reported the coating eliminated the need for traditional mineral-oil-based lubricants in that aggregate-handling application [S1]. The 2007 Vanhulsel space-mechanism work, by contrast, used ~50 at% hydrogen DLC on AISI 52100 angular-contact bearings and confirmed dry running in vacuum, with measured friction coefficients of 0.007 to 0.013 in vacuum, 0.02 to 0.03 in dry nitrogen, and 0.22 to 0.27 in humid air [S2].
Hydrogen Content Sets the Run-Dry Limit
The single biggest variable is hydrogen content. Highly hydrogenated DLC films at roughly 50 at% hydrogen delivered a vacuum friction coefficient as low as 0.008 in ball-on-disc screening on AISI 52100 substrates, and the same chemistry on angular-contact bearings produced low-torque vacuum life roughly 2x the benchmark MoS2 solid-lubricant coating [S2]. Hydrogen-free or low-hydrogen a-C:H grades (ta-C, a-C) behave differently: Piotrowska's belt-conveyor tests were conducted on a-C:H applied by PACVD and the abstract frames the coating as a way to extend grease-lubricated bearing life in abrasive stone-dust service, not as a wholesale grease replacement [S1].
Practical consequence: a spec sheet that quotes "DLC" without naming the hydrogen fraction, the deposition process, and the test environment is incomplete. Hydrogen-rich grades earn the "no grease" claim; standard a-C:H grades do not.
Grease Still Pulls Weight in 80-90% of Industrial Bearings
Grease remains the dominant bearing lubricant in roughly 80 to 90% of industrial bearing applications, used for its ability to stay in place, reduce maintenance, and seal against contaminants [S5]. DLC is a complement, not a wholesale substitute: it raises wear resistance in boundary and mixed lubrication where the grease film thins out, and it tolerates the starved-lube conditions that cause most premature grease-lubricated bearing failures [S4][S5].
NHBB's hard-coating reference states explicitly that DLC and tungsten-DLC (WCC) provide increased bearing performance "when the factors responsible for life-limiting bearing wear are too extreme for standard oil- or grease-lubricated bearings", with both coatings deposited by PVD or CVD to a depth of up to 5 micrometers [S4]. That wording is deliberate: extreme service, not normal service, is the design driver. Over-greasing remains a failure mode even with a DLC underlayer; excess grease still raises friction and heat [S5].
Where DLC Runs Dry: Vacuum, Inert Gas, Specialty Medical and Space
Dry-running DLC makes sense in three well-defined pockets: [S3]
1. Space mechanisms, where liquid and grease lubricants outgas, migrate, or freeze. Hydrogenated DLC angular-contact bearings generated low torque after a running-in period in vacuum and survived in-air ground testing without destroying the in-space performance, an advantage MoS2 cannot match [S2].
2. Inert or dry-nitrogen environments, where the coating measured friction coefficients of 0.02 to 0.03 and stayed chemically stable [S2].
3. Medical and food-grade hardware, where DLC and WCC are specified for biocompatibility, corrosion resistance, and repeated autoclave cycles, with the coating handling lubrication duty in cycles where grease would contaminate the process or degrade under sterilisation [S4].
For a general-purpose ball bearing running in humid air at moderate speed, expect DLC to be paired with grease, not to replace it.
Coating Thickness, Adhesion and the Flaking Failure Mode
Coating thickness is constrained by bearing geometry. NHBB's reference caps DLC and WCC at 5 micrometers via PVD or CVD so the coating can be specified without resizing the rings or rolling elements [S4]. Thicker films risk spalling under Hertzian contact stress; thinner films lose wear life. The Malloy wind-turbine field report documents the failure mode in plain language: DLC on main-shaft rollers spalled and flaked off once the coating was damaged, with the failures concentrated on larger platforms where the lubricant film was already failing [S3].
Adhesion is the second constraint. WCC is described as more chip-resistant than plain DLC at high speed, because the tungsten carbide phase toughens the interface [S4]. Hydrogen-rich DLC on bearing steel (AISI 52100 / 100Cr6 class) is the substrate pairing consistently reported in peer-reviewed work [S1][S2].
Selection Rules: Grease or Dry, by Service Condition
Use this decision check before specifying a DLC-coated ball bearing: [S1]
1. Environment: vacuum, dry N2, or sterile medical room? Hydrogenated DLC can run dry or lightly greased. Humid ambient air? Plan on grease [S2][S4].
2. Lubrication access: sealed-for-life bearing with no relube path? A high-hydrogen DLC film is one of the few realistic dry-lube options. Re-greasable bearing? Keep the grease and let DLC handle boundary-lube excursions [S2][S5].
3. Contamination load: abrasive particulate, water ingress, stone dust? DLC plus grease is the proven combination from the Piotrowska aggregate-industry study; grease carries the contaminant away and DLC protects the raceway [S1].
4. Speed and temperature: high speed or high temperature? Tungsten-DLC is the tougher choice for chip resistance; pair it with a high-temperature synthetic grease if the environment is oxidising [S4][S5].
Failure Modes That Override the "No Grease" Promise
Three documented failure paths still apply even with a DLC layer. First, coating delamination: DLC flaking has been observed on wind-turbine main-shaft rollers, with the failure mode shifting from gradual wear to sudden spall once the film is breached [S3]. Second, grease starvation: if the relube interval is missed, the DLC buys time but does not replace the hydrodynamic film the grease was providing, and pitting still develops on the rolling surfaces [S1][S5]. Third, over-lubrication: pumping in more grease than the bearing cavity can hold raises churning torque and case-hardening temperatures, shortening life regardless of the coating [S5].
For more on the mechanics of stacked dry-film contact pairs, see this analysis of disc spring parallel stack friction hysteresis damping values, which uses the same 5 micrometer-class film thickness discipline applied to a different sliding interface.
Limits of the Evidence and What to Track Next
All five sources converge on a single point: DLC is a coating that extends bearing life in poor-lubrication regimes, not a universal oil-and-grease replacement. The 2007 Vanhulsel dataset is the cleanest run-dry evidence base and still underpins current space-mechanism DLC practice [S2]. The 2025 Piotrowska paper is the freshest peer-reviewed evidence for grease-plus-DLC in aggregate and bulk-handling service, and is the right citation for any industrial specifier who needs to justify keeping the grease gun [S1]. Watch for two signals over the next 6 to 12 months: follow-on Piotrowska-group papers quantifying grease interval extension on DLC-coated conveyor bearings, and OEM datasheets that finally publish hydrogen-content and deposition-process callouts alongside the generic "DLC" label.
Spec-level background on the components involved: ball screw, and ball spline.