Originally Posted By: Gokhan
The history and mechanisms of ZDDP
H. Spikes (Tribology Section, Department of Mechanical Engineering, Imperial College, London SW7 2AZ, UK)
Tribology Letters, Vol. 17, No. 3, Pages 469 - 489, October 2004
Excerpts from the ZDDP review paper:
1. Introduction
Zinc dithiophosphates (ZDDPs) are arguably the
most successful lubricant additives ever invented. They
were introduced over 60 years ago, have been in continuous
use ever since and are still being employed in
practically all current engine oils. This longevity is all
the more striking since strenuous efforts have been
made by additive companies over the last 10 years to
replace them, but in vain. It has so far proved impossible
to identify any reasonably cost-effective compound
having comparable antiwear performance to ZDDPs
in engine oils.
As well as being remarkable in their performance,
ZDDPs have also been astonishingly successful in their
ability to inspire research. The last half century has
seen an extraordinary number of published research
papers describing investigations of how these additives
behave in their triple role as antioxidants, corrosion
inhibitors and antiwear agents.
...
It is not yet clear whether the limits of phosphorus
and sulphur in engine oils will be reduced further in
future, leading perhaps to the eventual disappearance
of ZDDP. Recently, attention has started to focus on
the possibility of replacing a blanket limit on the level
of phosphorus and sulphur in engine oils to a measure
that better reflects the tendency of these elements to
volatilise and thus reach the after-treatment catalyst
[15]. This may eventually lead to a limit on P- and
S-containing additive volatility or to a test which monitors
these species in the exhaust and thus permits
imaginative new formulations based on low volatility
additives [13]. Whatever the future however, there is
no doubt that the slow pace of reduction of phosphorus
levels in engine oil specifications over the last
5 years reflects the remarkable effectiveness of ZDDP
as an antiwear additive, and the great difficulty that
additive companies have had in finding a replacement
with comparable performance.
...
4.8. Antiwear properties of ZDDP
When considering the mechanisms by which ZDDP
prevents wear, it is important to note that ZDDP is
both an antiwear and a mild EP additive, i.e. it both
reduces wear and also inhibits the onset of scuffing.
This was recognised in the 1960s, when the influence
of metal type and alkyl group structure on wear and
EP behaviour were measured and compared [35,36].
Antiwear effectiveness was found to correlate inversely
with thermal stability of the ZDDP but this trend was
less clear-cut with respect to EP effectiveness [35]. Several
studies have suggested that the antiwear behaviour
of ZDDP results from its ability to form a phosphate
film while its EP response results from its ability to
form iron sulphide [53,130]. This is consistent with
other antiwear and EP additives; sulphur-free phosphorus
additives are often effective antiwear but generally
ineffective EP additives, while organic sulphides,
although possessing some wear-reducing capability are
generally regarded as EP additives [131,132]. Similarly,
studies have shown that in mild rubbing conditions the
surface film present is mainly a thick phosphate film
but that in severe, heavily loaded/high sliding speed
conditions a much thinner film with high sulphur content
is formed [53]. Thus we need to distinguish
between the ‘‘mild-wear’’ and ‘‘severe wear’’ action of
ZDDP, the latter being essentially an EP response.
This EP aspect will not be discussed in detail in this
review except to note that studies of thermal degradation
of ZDDP have shown that most of the sulphur
present in these molecules is converted to oil-soluble
organic sulphides and disulphides and that these are
well-known EP additives.
From the literature, three main ways that ZDDP
acts as an antiwear agent have been proposed; (i) by
forming a mechanically protective film; (ii) by removing
corrosive peroxides or peroxy-radicals; (iii) by
‘‘digesting’’ hard and thus abrasive iron oxide particles.
Each of these is discussed briefly below.
The most generally accepted view of ZDDP antiwear
action is that the reaction film acts simply as a
mechanically protective barrier [133]. This prevents
direct contact and thus adhesion between metal or
metal oxide surfaces and may also operate as a cushion,
reduces the stresses experienced by the asperity
peaks of the metal substrate. The relative importance
of these two effects has not been determined. With this
type of antiwear action, once a ZDDP film forms,
practically all that wear that subsequently occurs is
presumed to be that of the ZDDP film itself. (In fact,
ZDDP tribofilms appear to be very resistant to wear
and several studies have shown that once formed they
H. Spikes/The history and mechanisms of ZDDP 483
are only very slowly worn away even when the ZDDPcontaining
oil is replaced by a base oil [89,103]). In
this case, in mild wear conditions, the only loss of substrate
may be from iron oxide which has reacted to
form a phosphate film.
The second proposed mechanism of the antiwear
action of ZDDP is that it reacts with peroxides in the
lubricant; thereby preventing these from corrosively
wearing the metal surfaces present [69,70]. This mechanism
was convincingly demonstrated by both Habeeb
and Rounds in the 1980s and no subsequent work has
challenged it.
The third suggestion is more controversial. Martin
and colleagues have proposed that iron oxide particles,
that would cause abrasive wear, embed in the
ZDDP antiwear film and are ‘‘digested’’ to form
relatively soft iron phosphate, thus negating their
harmful pro-wear effect [62,64,129]. This model
appears to have been inferred from identification of
iron phosphate in wear particles and in the rubbing
track rather than any direct evidence of iron particle
digestion. It does seem likely that iron oxide from
the metal substrate diffuses into the ZDDP reaction
film to replace some of the zinc cations with iron ones
and form iron phosphates, and recently SIMS depth
profiling has been used to show that there is a much
lower iron oxide concentration beneath the ZDDP
tribofilm that on the surrounding metal surfaces
[96]. What is lacking as yet is direct evidence that
harmful iron wear particles are removed by a digestion
process.
One interesting aspect of ZDDP wear performance
that has arisen very recently is that ZDDPs appear to
strongly promote micropitting wear. Micropitting
results from localised plastic deformation due to the
surface loadings resulting from rolling/sliding asperity
contact and it has been shown that ZDDP, because it
very rapidly forms a protective film, prevents or postpones
effective running-in of rough surfaces. This leads
to high asperity stresses being maintained and consequent
micropitting [134]. What is not yet clear is the
extent to which this is an undesirable feature of all antiwear
additives or of ZDDPs in particular.
